Port as int, default host in config and separate implementation of handler

[FirstL1ght]

• Port in rcon.toml is now int, but string is also supported.

• RconServer now takes a RconHandler object (this is not the old RconHandler which was renamed to RconChannelHandler). RconHandler contains the command handling implementation and also methods such us onBind, onClientConnected, onClientDisconnected and onShutdown which are called by RconServer.

Tested on Velocity versions 3.1.1-98 and 3.1.2-SNAPSHOT-184