- Added a helper function, ZODB.blob.is_blob_record for testing whether
  a data record is for a blob.  This can be used when iterating over a
  storage to detect blob records so that blob data can be copied.

In the future, we may want to build this into a blob-aware
  iteration interface, so that records get blob file attributes
  automatically.

- Added the IBlobStorageRestoreable interfaces for blob storages
  that support recovery via a restoreBlob method.

- Updated ZODB.blob.BlobStorage to implement
  IBlobStorageRestoreable and to have a copyTransactionsFrom method
  that also copies blob data.

Also removed the version argument from the history method.