Save tombstones during partial merges. Fixes basho/bitcask#82

I decided to take a hack at this.  I don't actually know Erlang, but Bitcask's code is pretty clear.  I'm not super-happy with my solution for a couple reasons:  First, because there are actually some cases where it is safe to remove tombstones during partial merges, and second, because I think there is still more cleanup that could be done.  For instance, del_keydir is probably not necessary at all.  But I do think that this is probably an improvement in correctness.