* Migrate to LevelDbVersionKey
* Migrate to LevelDbMutationKey
* Migrate to LevelDbDocumentMutationKey
* Migrate to LevelDbMutationQueueKey
* Migrate to LevelDbTargetGlobalKey
* Migrate to LevelDbTargetKey
* Migrate to LevelDbQueryTargetKey
* Migrate to LevelDbTargetDocumentKey
* Migrate to LevelDbDocumentTargetKey
* Migrate to LevelDbRemoteDocumentKey
Also:
- Remove FSTLevelDBKey and tests
- Remove StringView and tests