BerkeleyConfig: Clarify in the docstring the semantics of the various
autopack variables, w.r.t. the differences between Full and Minimal
autopacking (the latter always does a gc pack because there's nothing
else it /could/ do :).

__init__(): We can create the packlock here, allowing us to get rid of
the __init__'s in the derived classes.

_withtxn(): Turns out to be useful for both storages.