add "synced_context_id"

the plan is to allow non-current-players to send require_random.
make server read "context_id"
every client asks server for random seed.

this breaks mp compability.
count requests instead of synced contexts for require_random
comment fix
Merge pull request #334 from gfgtdf/wesnothd_sides

allow every client to ask for random seed.

assume this scenaio:

[scenario]
## human sides 1,2
  [event]
    name = prestart
    [message]
      speaker = narrator
      message = "Hello"
    [/message]
    {GENERIC_UNIT .....}
  [/event]
[/scenario]

previously, in this scenario client 2 would have to wait for side 1 to click away the [message] because of the random seed required by {GENERIC_UNIT ..} because only client 1 was allowed to ask the server for a random seed (Note that waiting during prestart event means black screen), this patch is intended to fix such situations.