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/notification_test.go

@@ -523,7 +523,7 @@ func connectFakeWebSocket(t *testing.T, th *TestHelper, userID string, connectio
 		Locale:       "en",
 		ConnectionID: connectionID,
 	}, th.App, th.App.Channels())
-	th.App.Srv().Platform().HubRegister(webConn)
+	require.NoError(t, th.App.Srv().Platform().HubRegister(webConn))
 
 	// Start reading from it
 	go webConn.Pump()