cleaned up on disconnect.

Also avoid unnecessary iterator_gc calls to the server when there are
no iterators to be cleaned up.

doctests and for wating for a client to disconnect or connect.

when there is just one storage.

running layers in separate parallel processes.

runner doesn't call layer teardown when a layer is run as a
subprocess.  This seems insane, ...

requires passing a test object.

With this, it appears that I can run tests in parallel without getting
extra spurious errors.

Maybe this will reduce the chance if spurious test failures due to
addresses already in use.  Unfortunately, there's still a race that
may make running tests in parallel impractical until it's addressed.

ability to run layers in parallel.  This reduces test run time my more
than 75% for me.

leaving a thread behind.

an allocated block so that the first free block is right after the
last allocated block so that, on restart, we always continue writing
after the last written block.

size of an existing cache file.)

This means that cache files created by ZODB 3.8 can be read.

using ZEO Client Storages.  Just call ZEO.DB with the same arguments
you would otherwise pass to ZEO.ClientStorage.ClientStorage::

import ZEO
  db = ZEO.DB(('some_host', 8200))

to avoid unnecessary calls to the persistent_id hook for objects that
we know can't be persistent.

new_oids to avoid a race.

  twice (with test)
- remove irrelevent and unused test_suite definition from testDemoStorage.py

datetime modules. :(

  keyword option can be passed to the pack method.  A false value
  prevents garbage collection.

- The FileStorage constructor now provides a boolean pack_gc option,
  which defaults to True, to control whether garbage collection is
  performed when packing by default. This can be overridden with the
  gc option to the pack method.

The ZConfig configuration for FileStorage now includes a pack-gc
  option, corresponding to the pack_gc constructor argument.

- The FileStorage constructor now has a packer keyword argument that
  allows an alternative packer to be supplied.

The ZConfig configuration for FileStorage now includes a packer
  option, corresponding to the packer constructor argument.

Added a new test class for testing packing without gc.

Moved some useful tests to PackableStorage so they are run by storages
without undo.