misshandles URLs containing semicolons (path-segment parameters.

None.  If a non-resolvable conflict error occurs, we try to coerce
serial to something more humanly readable, but U64() can't be passed a
None, so let's be a bit more robust.

the environment.  No type checking is done, except that if env is not
a string or not None, then it's the caller's responsibility to pass in
a valid DBEnv.

that a pickle could contain references to objects we don't know
about (i.e. that isn't in our _serials table).  I'm not completely
sure why this could happen, but this makes pack() a little more robust
in the face of such anomolies.

This broke at least WebDAVs MOVE and COPY operations.

1.0 release.  These changes pass all the tests, but the proof will be
packing some real data (to come next -- this stuff has to be checked
in first).  Specifically,

__init__(): We now allocate a pack-lock which is acquired in pack() to
prevent multiple threads from packing at the same time.  This should
be deadlock-proof because you should never have the storage lock when
you want to acquire the pack lock (but I could be missing something
obvious).

_rootreachable(): New method which calculates the set of object ids
reachable from the current revision of the root object.

_zapobject(): Rewritten to remove the recursion.  It populates a
dictionary (passed in the parameter) with the oids for any objects
whose refcounts went to zero because of the object we're now zapping.

_zaprevision(): Same deal as _zapobjects().

_dopack(): The guts of the pack() operation, done this way so pack()
itself can just be a wrapper around _dopack() with pack lock
acquisition/release.  This also does not acquire the storage lock
until it actually has to start zapping revisions, and it releases and
reacquires the lock for each revision it zaps.  This gives other
threads an opportunity to do work during a pack, which should never
impact the pack as any work they do will happen in the future (and
we're safe against bogus future pack times).

pack(): Acquire the pack lock, _dopack(), release the pack lock.

Also, move the opening of the output and log files above the opening
of the storages so there's a closer link between that and the
try/finally that closes closes the storages.

will do this.

"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 <>