add functions in replay
use synced_context

 the main intention of that patch is to fix gna.org/bugs/?20871, gna.org/bugs/?21697, some OOS erros related to use of multiple
[get_global_variable] with different sides in one event.
To remove the deterministic random for traits and similar as discusssed
here http://forums.wesnoth.org/viewtopic.php?f=6&t=39611. And to sync
start end prestart events.

this patch unifies how side commands (like attack, recruit, ...) are
executed, during human turn, replay or ai turn by makeing them al call
the same function scned_context::run_in_scned_context.
We also use the new random generator which is automaticly synced during
the execution of scned_context::run_in_scned_context, and otherwise not.
So we cannot cause oos anymore by using rand= from an unsnced command
like "select"
Alternativeley we can also set the new random synced by using the new
RAII object set_sconext_sycned, this happens for example during prestart
events.
Unlike the old random generator, the new random generator is not
determinstic by default because it asks the server (or itself in a SP
game) for a new seed, at every side command (attack, move, recruit ...)
that requires random ,similar to how the previous rand for attacks
worked (it doesnt ask at the beginning, just the first time it is needed
unlinke how attacks worked before). That way we can also unify the rng
for attacks and for other random choices.

this commit breaks ai's advancement aspects, but form what i have heared
that should be fixable.
this commit might create some unused code, that shall be cleared,
especialy i want to remove random.hpp/cpp, rng.hpp/cpp
fix
use synced_context in moves

on order to get the same move results that we got during the game in replay, we now save continue_move/skip_sighed in the replay.
then we also call move_unit with replay_dest = NULL, is_replay =false during replay so that in replay the same code is executes as in the original game, that also means that all the code in move.cpp that the variable replay_ in move.cpp is now always false. which means there is some unused code now in move.cpp
i still hesitate to remove this code because it somehow seems to be too easy to do it like this, but i have found no bugs.
i suppose, that is_replay_ if a leftover from a time when vilibility wansn't calculates normaly during replays.
in a later patch i also put skip_ally_sighted in the replay data.

i don't use run_in_synced_context for moves because some methos use the returnvalue of move_unit.
use synced_context in recruits/recalls
add third argument to recorder.user_input
remove unused replay.cpp::type_by_index
use synced_context in undo and rest
add from_side check for require_random

"from_side" is an attribute whose only purpose is to enable a check by
the server that a package is really sended by the side given in
"from_side", if thats not the case, the server sets an attribute
"side_invalid". The main purpose of this contruct is to disallow
cheating in mp.
adding wesnoth.is_synced() #21697
removed unused rng code.

i also replaced set_random_results/get_random_results with
synced_checkup in recruit_checksums in create.cpp.

i don't see a reason why we shouldn't use the checkup when calling
place_recruit from wml, since the wml is called on all clients. Just
like a normal recruit. (if we are in an unsynced context then checkup
doesnt have any effect anyway.)
stop replay when we have reached the end.

the old code used to execute one last [end_turn] after the end of the
replay was reached, This doesn't work because the wml might require
[message][option] or similar during "turn end" events and we don't have
the data for that.
bring back unit_advancements_aspect

by moving code from synced_commands.cpp to actions/attack.cpp so that it
can be directly called from ai/actions.cpp
removed unused code
added a check for unit's destination in moves
correct the value of pos_ in replays.
rearrange comments and debuglogs
add skip_ally_sighted to replay

because this can affect the result of a move, we need this data in the
replay.
add comment in replay.hpp
removed unused code