MM-61130: Use a channelMember map at web_hub level (#28810)

Tests at very high scale indicates that the iteration
of all connections during websocket broadcast starts
to become a bottleneck.

To optimize this, we move the channelMember cache from
inside web_conn.go to the hubConnectionIndex.

This involves adding a new map keyed by the channelID
and containing all webConns where the user is a member
of that channel. Subsequently, a new method needed to
be added to invalidate the cache which previously
used to happen in web_conn.

And as a last step, we remove the cache from web_conn
to reduce SQL queries to the DB.

https://mattermost.atlassian.net/browse/MM-61130

```release-note
NONE
```

server/channels/app/platform/websocket_router.go

@@ -49,6 +49,7 @@ func (wr *WebSocketRouter) ServeWebSocket(conn *WebConn, r *model.WebSocketReque
 
 		session, err := conn.Suite.GetSession(token)
 		if err != nil {
+			conn.Platform.Log().Warn("Error while getting session token", mlog.Err(err))
 			conn.WebSocket.Close()
 			return
 		}
@@ -56,7 +57,12 @@ func (wr *WebSocketRouter) ServeWebSocket(conn *WebConn, r *model.WebSocketReque
 		conn.SetSessionToken(session.Token)
 		conn.UserId = session.UserId
 
-		conn.Platform.HubRegister(conn)
+		nErr := conn.Platform.HubRegister(conn)
+		if nErr != nil {
+			conn.Platform.Log().Error("Error while registering to hub", mlog.String("user_id", conn.UserId), mlog.Err(nErr))
+			conn.WebSocket.Close()
+			return
+		}
 
 		conn.Platform.Go(func() {
 			conn.Platform.SetStatusOnline(session.UserId, false)