PLT-4430 improve slow channel switching (#4331)

* PLT-4430 improve slow channel switching

* Update client side unit tests

* Convert getChannelsUnread to getMyChannelMembers and address other feedback

* Pull channel members on websocket reconnect

webapp/utils/constants.jsx

@@ -73,6 +73,7 @@ export const ActionTypes = keyMirror({
     RECEIVED_CHANNEL: null,
     RECEIVED_MORE_CHANNELS: null,
     RECEIVED_CHANNEL_STATS: null,
+    RECEIVED_MY_CHANNEL_MEMBERS: null,
 
     FOCUS_POST: null,
     RECEIVED_POSTS: null,