This makes it easier to add checks in handlers, which should improve security over time.
I think this approach is more readable and testable than calling Client#disconnect straight up,
while it also decentralizes the handling.
Split into 1 LoginServer and 1 ChannelServer per channel.
There is still a lot of cleanup and refactoring to be done.
Currently, the reliance on IoSession holding client state
is the most pressing issue to be addressed.
