optimize getUnreadByField

only get the most recent 100 notifications
dont call UserNotifications.getNotifications which does alot more work,
call the db directly
Merge branch 'master' of https://github.com/NodeBB/NodeBB
added nid
cleanup and optimize notifications
user onlineUsers instead of calculating from rooms
emit stats to registered users only
fix notifications to actually get recent
dont search empty string, get 20 tags instead of 10
closes #2278
#2278
fix browsing users removal
closes #2276
closes #2200
#2218
updated master to report 0.5.3-dev