three new configuration variables for controlling how autopacking
works.  Basically, you set an autopack frequency, a "packtime" -- the
point in the past you want to pack to -- and a counter for
automatically doing a classic pack.

Specific changes here include:

_setupDBs(): If autopacking is enabled, create the autopacking thread
object and get it started.

close(): When shutting down the storage, we need to stop and join the
autopacking thread, otherwise I think we have a high possibility of
corrupting our database (requiring recovery).

_dopack(): Add a flag for whether full gc should be done or not.
That's about the only difference between classic pack and autopack
(the latter does not do gc).

autopack(): The method that the autopacking thread calls to start an
autopack.  It takes a pack time with the same semantics as pack(), but
it also takes a flag specifying whether to do garbage collection of
unreachable objects or not.

_Autopack: A derived class of threading.Thread to handing the
background autopacking.