- Change protocol to use SHA1 for all checksums:
  - Use SHA1 instead of CRC32 for data checksums.
  - Use SHA1 instead of MD5 for replication.

- Change DatabaseManager API so that backends can store raw data separately from
  object metadata:
  - When processing AskStoreObject, call the backend to store the data
    immediately, instead of keeping it in RAM or in the temporary object table.
    Data is then referenced only by its checksum.
    Without such change, the storage could fail to store the transaction due to
    lack of RAM, or it could make tpc_finish step very slow.
  - Backends have to store data in a separate space, and remove entries as soon
    as they get unreferenced. So they must have an index of checksums in object
    metadata space. A new '_uncommitted_data' backend attribute keeps references
    of uncommitted data.
  - New methods: _pruneData, _storeData, storeData, unlockData
  - MySQL: change vertical partitioning of 'obj' by having data in a separate
    'data' table instead of using a shortened 'obj_short' table.
  - BTree: data is moved from '_obj' to a new '_data' btree.

- Undo is optimized so that backpointers are not required anymore to fetch data:
  - The checksum of an object is None only when creation is undone.
  - Removed DatabaseManager methods: _getObjectData, _getDataTIDFromData
  - DatabaseManager: move some code from _getDataTID to findUndoTID so that
    _getDataTID only has what's specific to backend.

- Removed because already covered by ZODB tests:
  - neo.tests.storage.testStorageDBTests.StorageDBTests.test__getDataTID
  - neo.tests.storage.testStorageDBTests.StorageDBTests.test__getDataTIDFromData