make server level_ reflect true start of game position

Previousy, the [side]'s in level_ were updated with controller changes,
so that observers who join would be up to date, since controller changes
were not stored in history. We now change this, so that controller changes
are stored in history.

These are stored as "[record_change_controller]" tags rather than
"[change_controller]" because during a replay we should not
restart the turn and reinit in the event of such a change.

This is in preparation to move all controller tweaks to server-side.
move controller tweaks to server

(fixup of 736ceaa6c7e81882c9c5b2e932307b1f1ecb3bfd)

Following discussion with Soliton, we move all "controller tweaks"
i.e. assignments of networked side to human on the matching client
at start of game, to be server decisions, performed concurrent with
start of game.

In fact the controller tweaks are performed using
game::change_controller, which is modified so that it doesn't try to
make server messages before the game has started.

When the server receives [start_game] from the host, the server will
call game::change_controller for every side, sending corresponding
messages to all players and observers, updating them as appropriate.
The server will also rewrite level_ so that human sides are "network"
and ai sides are "network_ai", making the level_ correct for any
observer who subsequently joins.

We remove the obsoleted client-side code in playcampaign.cpp,
multiplayer.cpp.

We also add to multiplayer_wait.cpp the necessary support code to
interpret [change_controller] messages recieved before game start.
update changelog
Merge branch '1.12' of https://github.com/wesnoth/wesnoth into 1.12