Prep ZODB 5.5.1 release.

Fix KeyError on releasing resources of a Connection when closing the DB.

Use the regular transaction manager wrapped by the thread-transaction
manager so we can call unregisterSynch when close is called from
another thread, typically during database shutdown.

Also, use a daemonic thread so that the test runner doesn't hand
waiting for the thread to die.

Prep 5.5.0 release.

Avoid most calls to time.sleep() in the tests by mocking time.[gm]time

grammar (typo)

Lifted from PR #164.

This happens on Python 3.7, evidinced by Appveyor (https://ci.appveyor.com/project/jimfulton/zodb/build/1.0.176/job/ehutnhnvbm28w5xl) but not travis because we run coverage on travis, and coverage disables refcounts of None.

This also makes us more deterministic. (Based on code I wrote for nti.testing.)

On my machine, this speeds up a sequential test run from 4:08 to 1:30.

We no longer get much benefit from running the layer tests in
parallel, in fact it looks like the overhead actually slows them down
slightly, so stop doing that.

p64 and p64 are faster and create more informative exceptions.

Fixes #216.

Add Python 3.7 support. 

And fix all the deprecations it reports.

Fixes #214 and fixes #215 (well there's one remaining deprecation in 'manuel')

Based on #217 so that all tests pass.

Remove support for 'python setup.py test'. [skip ci]

Fix tests with, and depend on, persistent 4.4.

Skipping CI because tests will fail until #217 is merged.

Fixes #218.

Fixes #213.

This is required to avoid a memory leak on Python 2.7.

See #203.

prefect -> prefetch

(Previously, protocol 2 was used for Python 2.)

  The zodbpickle package provides a `zodbpickle.binary` string type
  that should be used in Python 2 to cause binary strings to be saved
  in a pickle binary format, so they can be loaded correctly in
  Python 3.  Pickle protocol 3 is needed for this to work correctly.

- Object identifiers in persistent references are saved as
  `zodbpickle.binary` strings in Python 2, so that they are loaded
  correctly in Python 3.

Fix typo in doctest documentation of find_global method

* Don't seek on imports

(other than the possible seek for custom importers)

We were seeking to handle blob markers. This has two major drawbacks:

1. It wasn't possible to use a non-seekable file.  A use case for
   export/import is to copy database data.  An intermediate file, and
   associated I/O, could be avoided using a pipe, but pipes aren't
   seekable.

2. Seeks cause file-buffer data to be discarded, making IO far more
   expensive.

We didn't really need blob markers, because the preceeding blob data
records serve as markers.  (Now we're stuck with them for backward
compatibility.)

* Make cp's buffer size larger and configurable.

* Use the storage temprary directory when importing blobs

To avoid an extra copy.

Also, allow the copy (cp) buffer sie to be overridden on export.

(I see no obvious way to plumb it on import. :( )

* Oops, need to use a binary literal (Python 3)

* Respond to PR comments

Especially with regard to write capacity and search.

This hasn't been updated in a while and I feel that it is out of date.

I feel that this could use more work, but I'm anxious to do an
incremental improvement given the screencast I just released. :)

Report full oid of missing object during pack

Review Connection/DB closure