[DONTMERGE] serialize remoteCommand

Remotecommands updates are started in order, but are not garanteed to finish in order
We introduce a lock to make sure everything is serialized.

With buildbot nine, lots of the updates are doing asyncrhnous stuff like log update
and thus are definitly not finishing in order, thus a whole new class of weird behaviour.


PoC for discussion. dont merge it yet
I believe the lock should have a more explicit name as it's only about logs.