checkAbortVersionErrors
    checkModifyAfterAbortVersion
    checkCommitToNonVersion
    checkCommitToOtherVersion
    checkAbortOneVersionCommitTheOther

All are inhibited for Minimal storage.

(a.k.a. pickle pointer) is usually tid but not always!  In fact, it's
specifically not tid when we've aborted a version -- it'll be the
pointer to the pickle of the last non-version revision.  We were
actually setting up lrevid correctly, but writing tid to the metadata
record.  This fix writes lrevid instead.

storage has a failing abortVersion(), so it's mostly a checkpoint.

Not yet tested: commitVersion(), transactionalUndo(), undoLog().

Not yet implemented: history(), pack()

Specific changes:

    - Table txnOids renamed to txnoids everywhere.

    - Define a module global `zero' constant and use (via global)
      where ever appropriate (should be named ZERO but that's ugly).

    - _finish(): Only boost the refcount of objects referenced by the
      pickle when we've actually got a new pickle.  Also, add a note
      about the currentVersions table, which will contain multiple
      copies of vid/oid records.  It's easier to weed them out later
      than to weed them out here.  One of the places you need to weed
      them all out is at the end of this method, in the
      `delete-a-version' branch.

    - abortVersion(): Record oids found by scanning the
      currentVersions table in a set, to account for the possible
      multiple vid/oid records.  Fix some typos.  When calling
      write_nonversion_object() on the CommitLog, the prevrevid ought
      to be the revid of the current object revision, for undo
      purposes.

    - load(): Where we previously had a comment that asked whether we
      should assert that the version is empty (after we've checked
      everything else), now we really check that and raise a
      VersionError if not.

    - loadSerial(): Fix a typo.

    - store(): Inside the old-revision-id-is-true test, if the old vid
      is not zero and it is equal to the new vid, then the non-version
      revision of the object is the old non-version revision (added
      the else: clause).

    - transactionalUndo(): Implemented but not yet tested.

    - undo(): Removed.

    - versionEmpty(): Semantics are to return true if an unknown
      version is given.  As an extra bonus, if version is empty
      (i.e. the non-version) check the serials table.

common operation. ;)  Also, _dostore() automatically pickles the data
argument, since you don't really want anything else.

Lots of new tests:
    checkNonVersionModifiedInVersion()
    checkLoadSerial()
    checkVersionedStoreAndLoad()
    checkVersionedLoadErrors()
    checkConflicts()
    checkVersionLock()
    checkVersionEmpty()
    checkVersions()
    checkAbortVersion()

Skip for the Minimal storage:
    checkVersionedLoadErrors()
    checkVersionLock()
    checkVersionEmpty()
    checkVersions()
    checkAbortVersion()

Override for the Minimal storage (basically check for Unsupported exceptions):
    checkLoadSerial()
    checkVersionedStoreAndLoad()

long to make it more readable.

checkNonVersionStore(): Add some default arguments to make it useful
for checkNonVersionStoreAndLoad().

__init__(), close(): Remove self._references since we sniff the pickle
instead.

_decref() -> _zaprevision(): Also, changed implementation since we
observe that it can't really be shared with the _decref() for pack,
and we're moving to a non-destructive undo instead.  So make this
function (presumably) useful for pack() and cyclic garbage
collection.  We'll see...

undo(): Mark this TBR -- to be removed -- but don't (yet) remove it.

the Minimal and Full storages.

more code sharing.

DBHOME is unused, remove.

given by tpc_begin().

_finish(), _abort(): Fix typos.

TextMixin class.

storages, like transaction aborts and commits, changing objects, etc.
Doesn't test undo, versions, or packing.

"Using not needed variable"

object to fetch the object.

same set of tests that Minimal also passes).  Not yet tested:
versions, undo, packing, garbage collection.

InternalInconsistencyError: the base class should be AssertionError.

Fix typo in name of table that keeps the pickle reference counts (it
should be _pickleRefcounts).

next_object() => next()

Use referencef() in a completely backwards compatible way (I happen to
have an older version of ZODB installed).

load(): Fix typo in name of table that stores the pickle data (it
should be _pickles).

store(): Use a local variable called `zero' for the 8-byte zero
pointer.  When there is no current serial number for the object
(i.e. it is being stored for the first time), initialize oserial to
zero (but not orevid).  Better printing of the ConflictError message.
When there is no version string (i.e. this is being stored in a
non-version), initialize nvrevid to zero.  Better printing of the
VersionLockError.  Fix typo in return value; we should return the
cached serial number, not the serial number that was passed in as an
argument.