connection pools of remote stores can currently block. this is not a
problem when each request runs on a dedicated thread, but with async
code this is no longer true. if a remote store has exhausted all its
available connections on one executor and another job starts *on the
same executor* we'll deadlock if that job makes another remote store
request. unlike with sqlite previously it's not reasonable, not even
necessary, to make the pools unbounded: since we use pools only with
remote stores we can asyncify all of them at once, and since they're
leaves of all call stacks we do not have much code to change either.
Change-Id: I8c457e27893e22c2cfc35933307a5283c986805a
eldritch horrors
4e266a25d6