give do_replays return a better name

I forgot what my real intention with this change was, but i still think this is a good change, becasue now people don't have to look up the comments to see what do_replay returns.
So i committed it.
fix bug chat during action

when a player chats while multiple actions from another player are
executed on the local machine we before got an assertion error becasue
replay s add_command expects that there are no other commands yet to be
processed on the replay.
(this was broken at da4cdef146bf5a1b8026f12d6b1caa6c1cd20602)
we now fix this problem by ensuring that there is never more data on the
replay than needed.

for this purpose i wrote that playturn_network_adapter.cpp whose main
purpose it to split incoming [turn]s into smaller [turn]s
we also don't need the backlog anymore with this.