_zapobject(): Factored out functionality from _zaprevision(), this

method now just takes care of deleting all traces of a garbage
collected object, via oid.

_zaprevision(): Factored and rewritten to make logic more readable.
Leaves zapping of g.c.'d objects to _zapobjects().

pack(): In looping over the metadata, be sure to stop when we've hit
an oid different than the one we're looking for.  This caused way too
many metadata entries (and thus object) to be packed away.