admin: Remove locking; new safe client destruction.

admin: Remove locking; new safe client destruction.

The locking stuff is fiddly, and prevents cleanup of pending background
operations during destruction.  Instead, we add `destroyed' clients to a
list which we clean up just before going back to select.  This ensures
that they get cleaned up at a safe place, when there aren't functions
threaded on the stack which will be upset by the admin block vanishing
under their feet.