Store collaborators and onlineUsers in mongo
Re-fetch and populate board after joining
Fill in collaborators for all boards
Merge branch 'chore/move-board-users-to-mongo' into development
Save board online users when user leaves
Store online sessions, instead of users
Small refactor of socket handler code
Use socket to store and retreive session from boards
Added messaging when people join/leave the board
Don't emit join message back to user who joined
Send and save heartbeat
Expire stale sessions after five minutes
Better messaging
Re-join board via heartbeat if session cannot be found
Merge 'join' and 'heartbeat' messages
Merge branch 'chore/track-user-sockets' into development
Don't use cluster module any more.  Too hard.
No more CPUS env var
Some logging
Merge branch 'chore/remove-cluster' into development