Login response handler validation rework

[Z-Kris]

Changes the GameLoginResponseHandler#writeSuccessfulResponse function to return a non-nullable Session.

The old implementation performed certain checks in the writeSuccessfulResponse functions, which meant it could fail and return null. This however lead to some annoying server-side code, as servers had to allocate a slot in the world for the player, try to send it to the client, then continue on normally if it succeeded, and de-allocate if it failed.

The client type check has been migrated to occur during login block decoding - if it fails, the login block itself will not even be decoded (no RSA-decrypting etc), actually improving one potential attack vector.
The activity check has been removed as it happens in plenty of situations after it, and the connection limit has been migrated to its own function, meaning servers will have to implement it themselves now.