MM-29980: Optimize profilesInChannels cache to fast path (#16116)

* MM-29980: Optimize profilesInChannels cache to fast path

We add one more message type to the fast path- profiles in channels. There
are 2 primary reasons for this:

- This is not really a new model type, but just a map of users. And users already use
the fast path. So we can get some more gains without really investing much more code.
- A more important reason is that with the upcoming striped mutex changes, we will get
a higher throughput at the cost of a bit more CPU utilization. The reason being that
since less amount of time will be spent in lock-contention, the CPU is free to do more
stuff. So this change is to counter that increase.

As usual, this gives much better performance than the original decoder.

Micro-benchmark results
```
name               old time/op    new time/op    delta
LRU/UserMap=new-8    16.6µs ± 3%     3.9µs ± 4%  -76.15%  (p=0.000 n=10+10)

name               old alloc/op   new alloc/op   delta
LRU/UserMap=new-8    4.78kB ± 0%    2.74kB ± 0%  -42.65%  (p=0.000 n=10+10)

name               old allocs/op  new allocs/op  delta
LRU/UserMap=new-8      38.0 ± 0%      30.0 ± 0%  -21.05%  (p=0.000 n=10+10)
```

https://mattermost.atlassian.net/browse/MM-29980

Here are some results from a load test. The comparison is done with a 2 node cluster; one running master
and one running with this patch so that it's easier to compare. The total users are 2000.

<See PR>

```release-note
NONE
```

* Fix gofmt

* Trigger CI

services/cache/lru.go

@@ -221,6 +221,11 @@ func (l *LRU) get(key string, value interface{}) error {
 			_, err := s.UnmarshalMsg(e.value)
 			*v = &s
 			return err
+		case *map[string]*model.User:
+			var u model.UserMap
+			_, err := u.UnmarshalMsg(e.value)
+			*v = u
+			return err
 		}
 
 		// Slow path for other structs.

services/cache/lru_test.go

@@ -279,6 +279,17 @@ func TestLRUMarshalUnMarshal(t *testing.T) {
 	// This does not make an actual difference in terms of functionality.
 	u.Timezone = nil
 	require.Equal(t, user, u)
+
+	tt := make(map[string]*model.User)
+	tt["1"] = u
+	err = l.Set("mm", model.UserMap(tt))
+	require.Nil(t, err)
+
+	var out map[string]*model.User
+	err = l.Get("mm", &out)
+	require.Nil(t, err)
+	out["1"].Timezone = nil
+	require.Equal(t, tt, out)
 }
 
 func BenchmarkLRU(b *testing.B) {
@@ -434,6 +445,39 @@ func BenchmarkLRU(b *testing.B) {
 		}
 	})
 
+	uMap := map[string]*model.User{
+		"id1": {
+			Id:       "id1",
+			CreateAt: 1111,
+			UpdateAt: 1112,
+			Username: "user1",
+			Password: "pass",
+		},
+		"id2": {
+			Id:       "id2",
+			CreateAt: 1113,
+			UpdateAt: 1114,
+			Username: "user2",
+			Password: "pass2",
+		},
+	}
+
+	b.Run("UserMap=new", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			l2 := NewLRU(&LRUOptions{
+				Size:                   1,
+				DefaultExpiry:          0,
+				InvalidateClusterEvent: "",
+			})
+			err := l2.Set("test", model.UserMap(uMap))
+			require.Nil(b, err)
+
+			var val map[string]*model.User
+			err = l2.Get("test", &val)
+			require.Nil(b, err)
+		}
+	})
+
 	post := &model.Post{
 		Id:            "id",
 		CreateAt:      11111,