Put the tpc_begin() -> tpc_finish() in a try/except.  If an error
occurs, abort the current transaction and re-raise the error.

It appears that some storages (well, only Standby Storage) has trouble
shutting down if a transaction is in progress.

Replace three small integers with symbolic constants.
    TRANS_HDR_LEN = 23
    DATA_HDR_LEN = 42
    DATA_VERSION_HDR_LEN = 58
The constants make it easier to remember what kind of header is being
read and/or what kind of offset is being computed.

Refactor commitVersion():
    - Keep locking & arg checking in commitVersion(), but move actual
      commit logic to _commitVersion().  This removes a level of
      indentation.

    - Remove local vars that alias attributes of other vars

Add a few more POSKeyError exceptions.

Highlights:

BaseStorage.py:

Fix copyTransactionsFrom() when commitVersion(), abortVersion() or
transactionalUndo() is used.

FileStorage.py:

Add restore() method, which can store data records corresponding to
undo or version manipulations; add close() method to FileIterator
class; raise POSKeyError instead of KeyError for bad keys, to ensure
safe formatting of transaction ids (binary strings) in tracebacks.

POSException.py:

Add POSKeyError.

fsdump.py:

Deal with records indicating the undo or abort of a version doing
object creation.

tests/IteratorStorage.py:

New unittests for the iterator() method and interface of the
storage API; new unit tests for extended file iterators; new class,
IteratorDeepCompare; test of the iterator .close() method.

tests/testFileStorage.py:

Add class FileStorageRecoveryTest, which adds two simple tests for
copyTransactionsFrom().  This indirectly tests the new restore()
method.

1. The exception is in serial, not arg.
2. Must call handle_serials() before tpc_finish(), because
   handle_serials() may raise an exception that causes the transaction
   to abort.

The main test suite handles ZEO now.

Laredo.  [merging from Recovery branch into trunk]

Py_None.

Make sure that the test undoes an object creation and make sure that
the iterator for that transaction return None for the data attribute.

This will allow an iterator to handle aborted versions and undone
object creations.  XXX Still need a test for the latter.

XXX checkTransactionalUndoIterator() should too, but doesn't.

# XXX need a test case that covers iteration over a storage that
# includes a transactional undo that un-created an object.

If supplied, they are stored as metadata on the transaction object.
This is helpful for debugging.

It's not thread-safe to store a Transaction() object in an instance
variable without some sort of locking.  Nor is it desirable to use the
same transaction object for more than one transaction.

So get rid of all uses of self._transaction and replace with a local
variable.

running the rest of the tests in the child.

Let super tearDown() close the storage.

Tracking changes to ZODB/tests/StorageTestBase.

running the rest of the tests in the child.

The Standby-branch was branched from the StandaloneZODB-1_0-branch,
which includes the BTrees-fsIndex code.  I didn't include that change
in the merge, but everything else.  Terse summary follows:

BTreeModuleTemplate.c:
    Greg Ward's ConflictError patch

The Standby-branch was branched from the StandaloneZODB-1_0-branch,
which includes the BTrees-fsIndex code.  I didn't include that change
in the merge, but everything else.  Terse summary follows:

BaseStorage.py:
    Add read-only storage feature.
    Add TransactionRecord and DataRecord marker classes for iteration.
    Reformat some lines.

FileStorage.py:
    Add read-only storage feature.
    Greg Ward's ConflictError patch
    Reformat some lines.
    Add lastTransaction(), lastSerialno().
    Add bounds support to iterator().
    Use TransactionRecord and DataRecord.

Connection.py:
DemoStorage.py:
MappingStorage.py:
    Greg Ward's ConflictError patch

POSException.py:
    Greg Ward's ConflictError patch
    Add ReadOnlyError.

as argument