Introduce uninitialize for sessions

[aromaa]

Introduces uninitialize for sessions that is called when the player disconnects or the server is shut down. Currently handlers don't get any notifications when the mentioned events occur and are unable to restore any player state they are holding to.

For example, this can be seen when entering to a region in survival with a game-mode creative flag, disconnecting and waiting for the session to timeout. When connecting back to the server and walking out of the region you can see that the player is not switched back to survival as they normally would be.