admin socket: allow multiple pending requests
Right now we have a mutex that is held while processing an admin socket interaction (and possibly a bunch of other stuff making this hard; I don't know). It would be very helpful if we could have concurrent admin socket commands in flight.
At the moment this is motivated by debugging a command I'm writing (which is broken) and wanting to get diagnostic information out. But as we add commands which take a noticeable amount of time to run (eg, requiring disk accesses...) it becomes more important that we be able to issue such commands in parallel, or that diagnostic monitoring systems still be able to get out their data, or whatever.