2.4 and 2.5.

and 2.5.

Added APIs for removing committed blob data that makes files writable
before removing them and updated code that removes committed blobs or
blob directories to use them.

stack traces.

spurious failures.

that it is specific to blobs. Added logging when we have to copy.

willing to copy when renaming fails.

savepoint storage.

Also added a test for the normal blob case.

if we can't link.

Added support for copying to blob cache when renaming fails.

committed file name.

Make sure that uncommitted data gets cleaned up if w blob is GCed
without committing.

 - some cleanups
 - re-introduced a way to ask for a rename() operation but fall back to copy
   if rename doesn't work
 - using the rename_or_copy for blobs where reasonable
 - fixed a test that was trying to test a failing consumeBlob but where the
   wrong exception was tested for

   new uncommitted data.
 - Made blobs use the temporaryDirectory() method of storages to store new
   uncommitted data near the committed data.

Removed the soon to be replaced readDetached,

Hack os.rename rather than the blob to avoid a testing artifact in the
blob implementation.

- Unwritten blobs can now be read, and are empty.

- Blobs are considered modified when opened for writing.  This is a
   little bit more conservative than before but fixes a bug that a file
   opened with 'w' actually does modify the file and wasn't considered
   to be a change before.

- Optimistic savepoints now work.

- Fixed bug: could open multiple files for writing.

- Fixed bug: aborting a transaction removed uncommitted data for
   uncommitted blobs.

Todo: 
   Need to remove uncommitted data file if a blob is GCed even when a
   transaction isn't aborted or when it hasn't been added to anything.

- No-longer close files on transaction boundaries.

This allows us to get rid of the transaction-manager dance.

object is a blob.  YAGNI support for multiple blob implementations --
at least for now.

   closed connections.

away -- even before they are placed in the cache.  This causes a
problem if the object's state has a (direct or indirect) reference to
it. Added a pre-cache that allows the connection to return objects
alreadey being loaded when necessary.