Hide lagwarnings until the turnmanager waits for that client

[elexis]

The purpose of the lagwarnings is to notify players of who is slowing down the gamespeed. Every client triggers these when rejoining, thus making players complain about that client, even if the lag doesn't affect the game until the rejoin is complete (or at least until the client starts synchronizing). Since the netserver knows whether that client finished the rejoin, it should decide to postpone the lagwarnings until they become relevant.

Refs #3700 #3264, furthermore #4036 sometimes occurs with the lagwarnings too