Optimize TotalUnreadThreads (#18788)

We remove the left joins with inner joins and also
remove the coalesce on posts.DeleteAt which improves
the join plan from a nested loop to a hash join,
and also allows the DeleteAt to be used as part of the
index filter.

Old plan

```
 Aggregate  (cost=666288.49..666288.50 rows=1 width=8) (actual time=5114.328..5119.991 rows=1 loops=1)
   ->  Gather  (cost=2880.69..666288.27 rows=87 width=8) (actual time=614.214..5119.575 rows=533 loops=1)
         Workers Planned: 2
         Workers Launched: 2
         ->  Nested Loop  (cost=1880.69..665279.57 rows=36 width=8) (actual time=607.821..5099.078 rows=178 loops=3)
               ->  Hash Join  (cost=1880.28..664932.93 rows=229 width=16) (actual time=224.374..1494.698 rows=1866781 loops=3)
                     Hash Cond: ((posts.channelid)::text = (channels.id)::text)
                     ->  Parallel Seq Scan on posts  (cost=0.00..662980.29 rows=27568 width=43) (actual time=201.053..1013.362 rows=4410819 loops=3)
                           Filter: (COALESCE(deleteat, '0'::bigint) = 0)
                     ->  Hash  (cost=1866.60..1866.60 rows=1094 width=27) (actual time=1.486..1.486 rows=1002 loops=3)
                           Buckets: 2048  Batches: 1  Memory Usage: 74kB
                           ->  Index Scan using idx_channels_team_id_type on channels  (cost=0.42..1866.60 rows=1094 width=27) (actual time=0.028..1.331 rows=1002 loops=3)
                                 Index Cond: ((teamid)::text = '8ywxyw9ocp8smxrmjzrkqhrdwe'::text)
               ->  Index Scan using threadmemberships_pkey on threadmemberships  (cost=0.41..1.50 rows=1 width=35) (actual time=0.002..0.002 rows=0 loops=5600344)
                     Index Cond: (((postid)::text = (posts.rootid)::text) AND ((userid)::text = 'tc3p1yqw67d8idcp3g98awexqe'::text))
                     Filter: (following AND (posts.createat > lastviewed))
                     Rows Removed by Filter: 0
 Planning Time: 2.005 ms
 JIT:
   Functions: 59
   Options: Inlining true, Optimization true, Expressions true, Deforming true
   Timing: Generation 9.905 ms, Inlining 122.509 ms, Optimization 293.764 ms, Emission 186.064 ms, Total 612.241 ms
 Execution Time: 5125.739 ms
```

New plan

```
 Aggregate  (cost=683129.17..683129.18 rows=1 width=8) (actual time=1650.746..1656.455 rows=1 loops=1)
   ->  Gather  (cost=3766.65..683085.55 rows=17450 width=27) (actual time=641.073..1656.086 rows=533 loops=1)
         Workers Planned: 2
         Workers Launched: 2
         ->  Hash Join  (cost=2766.65..680340.55 rows=7271 width=27) (actual time=617.096..1635.203 rows=178 loops=3)
               Hash Cond: ((posts.rootid)::text = (threadmemberships.postid)::text)
               Join Filter: (posts.createat > threadmemberships.lastviewed)
               Rows Removed by Join Filter: 6
               ->  Hash Join  (cost=1880.28..679334.08 rows=45742 width=16) (actual time=15.196..1242.832 rows=1866781 loops=3)
                     Hash Cond: ((posts.channelid)::text = (channels.id)::text)
                     ->  Parallel Seq Scan on posts  (cost=0.00..662980.29 rows=5513543 width=43) (actual time=0.028..775.118 rows=4410819 loops=3)
                           Filter: (deleteat = 0)
                     ->  Hash  (cost=1866.60..1866.60 rows=1094 width=27) (actual time=1.572..1.573 rows=1002 loops=3)
                           Buckets: 2048  Batches: 1  Memory Usage: 74kB
                           ->  Index Scan using idx_channels_team_id_type on channels  (cost=0.42..1866.60 rows=1094 width=27) (actual time=0.025..1.372 rows=1002 loops=3)
                                 Index Cond: ((teamid)::text = '8ywxyw9ocp8smxrmjzrkqhrdwe'::text)
               ->  Hash  (cost=823.41..823.41 rows=5037 width=35) (actual time=256.240..256.241 rows=5007 loops=3)
                     Buckets: 8192  Batches: 1  Memory Usage: 417kB
                     ->  Bitmap Heap Scan on threadmemberships  (cost=159.45..823.41 rows=5037 width=35) (actual time=253.011..255.255 rows=5007 loops=3)
                           Recheck Cond: ((userid)::text = 'tc3p1yqw67d8idcp3g98awexqe'::text)
                           Filter: following
                           Heap Blocks: exact=601
                           ->  Bitmap Index Scan on idx_thread_memberships_user_id  (cost=0.00..158.19 rows=5037 width=0) (actual time=0.627..0.627 rows=5037 loops=3)
                                 Index Cond: ((userid)::text = 'tc3p1yqw67d8idcp3g98awexqe'::text)
 Planning Time: 1.952 ms
 JIT:
   Functions: 83
   Options: Inlining true, Optimization true, Expressions true, Deforming true
   Timing: Generation 10.148 ms, Inlining 115.819 ms, Optimization 395.923 ms, Emission 244.130 ms, Total 766.021 ms
 Execution Time: 1661.791 ms
```

No special test has been done on MySQL since these are pretty
basic optimizations and should apply to both equally.

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4?c=c9aa17f3-b918-4a78-86aa-01562e418634

```release-note
NONE
```

store/sqlstore/thread_store.go

@@ -149,7 +149,7 @@ func (s *SqlThreadStore) GetThreadsForUser(userId, teamId string, opts model.Get
 	if !opts.Deleted {
 		fetchConditions = sq.And{
 			fetchConditions,
-			sq.Eq{"COALESCE(Posts.DeleteAt, 0)": 0},
+			sq.Eq{"Posts.DeleteAt": 0},
 		}
 	}
 
@@ -169,9 +169,9 @@ func (s *SqlThreadStore) GetThreadsForUser(userId, teamId string, opts model.Get
 	go func() {
 		repliesQuery, repliesQueryArgs, _ := s.getQueryBuilder().
 			Select("COUNT(DISTINCT(Posts.RootId))").
-			From("Posts").
-			LeftJoin("ThreadMemberships ON Posts.RootId = ThreadMemberships.PostId").
-			LeftJoin("Channels ON Posts.ChannelId = Channels.Id").
+			From("Posts, ThreadMemberships, Channels").
+			Where("Posts.RootId = ThreadMemberships.PostId").
+			Where("Posts.ChannelId = Channels.Id").
 			Where(fetchConditions).
 			Where("Posts.CreateAt > ThreadMemberships.LastViewed").ToSql()