Fix early failure in User#disconnectAll for multiple connections
The ascending loop over the connections array wasn't appropriate, given that Connection#destroy mutates that array.
Battle#resendRequest's argument is now a connection, rather than user
- The higher specificity saves bandwidth and suppresses a client crash when another user connection joins the battle.
Fix support for multiple connections in the same battle
The request would not be sent to the connection as it wasn't in the room.
Fix logs sent to the first connection of a user joining a room
The message reporting their join would be sent twice.
Prevent battles between offline users from starting
There is no reason to allow them, while doing so allows some race conditions where the first connection of a player joining a battle room doesn't receive their request.
Merge pull request #1379 from Slayer95/users

Fix several issues with users's joining and leaving