Add support for RocksDB (dramatic memory improvement with multiple databases)

[cjroth]

Calling getKeys loads all keys into memory and filters through them. This means that if there is other data in AsyncStorage that is unrelated to the current database, such as other databases, the memory load increases linearly with the number of keys.

If using RocksDB via https://github.com/tradle/react-native-async-storage-rocks, there is method to only load keys with a given prefix. I've found that this solves the memory issue entirely.

[mqtik]

No android version for this.

[mqtik]

I tried this pull request. It's throws a lot of conflicts with replication.