move next_underlying_unit_id

from savefiles toplevel to savefiles [snapshot]/[replay_start]/[carryover_sides_start]
previously this was handled specialy at scenario toplevel sometimes and in those tags at some other times.

accoringly to that change we now set n_unit::id_manager at play_controller s constructor.
don't transfer carryover in replays

carryover_sides contains no data that wasn't already merged into replay_start.
remove unneeded argument in play_replay_level
refactor carryover_info::transfer_all_to

the plan is to fully remove the [side] from [carryover_sides] in this
case, but it needs some testing still.
Merge pull request #200 from gfgtdf/gamestate_refactor

move next_underlying_unit_id + game_state refactor