"from ZODB import utils" -> "from ZODB.utils import p64, U64"

fixed comment-doc for pickles table

close(): Don't close the commitlog since the base class's close() will
do this now.

_zapobject(): Wrap _serials.delete() in a try/except ignoring
DBNotFoundError.

pack(): Compare refcounts to None by using "is not" instead of <>