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/web_hub.go

@@ -16,16 +16,6 @@ func (a *App) GetHubForUserId(userID string) *platform.Hub {
 	return a.Srv().Platform().GetHubForUserId(userID)
 }
 
-// HubRegister registers a connection to a hub.
-func (a *App) HubRegister(webConn *platform.WebConn) {
-	a.Srv().Platform().HubRegister(webConn)
-}
-
-// HubUnregister unregisters a connection from a hub.
-func (a *App) HubUnregister(webConn *platform.WebConn) {
-	a.Srv().Platform().HubUnregister(webConn)
-}
-
 func (a *App) Publish(message *model.WebSocketEvent) {
 	a.Srv().Platform().Publish(message)
 }