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

model/user.go

@@ -98,6 +98,12 @@ type User struct {
 	TermsOfServiceCreateAt int64     `db:"-" json:"terms_of_service_create_at,omitempty"`
 }
 
+//msgp UserMap
+
+// UserMap is a map from a userId to a user object.
+// It is used to generate methods which can be used for fast serialization/de-serialization.
+type UserMap map[string]*User
+
 type UserUpdate struct {
 	Old *User
 	New *User