Create MapDB based persistence provider

With the implementation of MODE-2528, ModeShape has a storage SPI which is focused around the SchematicDB abstraction and which really has a single requirement: that a persistence provider is transactional and is able to return a stored document based on a given ID. Also, a persistence provider should not worry about concurrent updates on the same key, since with MODE-2546 these cases should be handled by ModeShape.

Since we're already using MapDB in a couple of places: the repository journal and the local index provider and since MapDB meets the above criteria, we should add a second persistence provider which stores data in MapDB.

Since MapDB is not distributed, this store will not be able to be used when clustering, but for standalone cases it should offer superior performance to a standalone JDBC-based store.