added while a pack was stuck between a mark and a sweep, the mark
would not have found the new objects as root reachable (because they
wouldn't have been stored yet), but the sweep will find then and
erroneously delete them.

_setupDBs(): Add a _packing flag which gets set whenever we're doing a
classic or auto pack

_docommit(): When the _packing flag is set, copy all oids in the _oids
table to the _packmark table, so any objects added between the mark
and sweep phases will not be gc'd.  This is fine because if they're
still not root reachable by the next gc pass, they'll get collected
then.

pack(), autopack(): Set and reset the _packing flag around the calls
to _dopack().