Re-enable GC from publishers

It turns out that having publishers GC is pretty important. That
mechanism was largely responsible for keeping memory use down on the
backend Redis servers, and turning it off proved a catastrophe.
Always call destroyClient's callback

Since we want to move to a serial GC, the callback is now always
invoked, but with a "false" argument if the GC didn't complete for
whatever reason.
Rewrite GC to process continuously

This replaces the batch interval-based GC with a serial process. Now,
clients are processed one-at-a-time, and we depend heavily on the
callbacks (and errbacks) to re-invoke the GC loop. An experiment,
really.
Bump version
Merge branch 'publisher-gc'