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

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)

Start by locating the existing migration tool for the piece store and DAGstore to Piece Directory, then trace how it selects Piece Directory implementations. Define the migration path from LevelDB to Couchbase and verify whether the reverse path is required; done means the listed implementation migration works without data loss.

Written by the indexing model from the issue text.