Reintroduce command crashguard

If a crash happens during a command (which is around 99% of all
crashes), a bug in either Node or SockJS causes the user who issued
the command to be unable to do say or do anything (although they'll
still receive messages).

We used to have a crashguard to prevent this from happening, but
it was removed due to suspicions of being a significant lag source.
Later lag investigation has shown that it's not the biggest source
of lag, so we're reintroducing that crashguard for now.