Piece Directory migration tool - cb->leveldb ; leveldb->cb

[nonsense]

We need to be able to migrate from Couchbase to LevelDB and back.

At the moment we have a migration tool from the existing piece store and DAGstore to the Piece Directory, but not between various implementations of the Piece Directory

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• LevelDB -> Couchbase
• Couchbase -> LevelDB (we probably don't need this at all, as we don't foresee SPs switching from Couchbase to LevelDB at all)