Refactor users related code (mp::connect).
Re-enable lobby to show reloaded game titles.
Remove obsolete "simple server" related code (mp::connect).

As a result of this, 'connected_user' struct was no longer useful and
is now removed. Connected users information are now being hold in
std::set<std::string>, because only their unique usernames matter
in mp::connect.
Minor clean-up of users related code (mp::connect).
Allow non-reloaded games to use "Reserved" option.
Reserve sides for players who played them in previous scenario.

The information for the previous scenario's sides which have "save_id"
and a player ("player_id") assigned to them are now being recorded in
"mp_game_settings". This information is used in mp::connect for every
non-last scenario to reserve sides for previous players so that
they could easily join the game and be placed in the correct side.