no concurrent execution during arbiter restart

An explicit test is needed because the restart()
command is a kind of stop() command. The start after
the stop is made (externally) by circusd for plenty of reasons.

So the restart() command is over before the arbiter is really restarted.
So the classic synchronized decorator is released too early.

see #714
Merge pull request #718 from thefab/fix714

Fix #714