All TieredDatastore functions should be implemented

The main purpose of this module is to be used with [TieredDatastore](https://github.com/ipfs/js-datastore-core/blob/master/src/tiered.js). Accordingly, all the methods that may used by this Datastore should be implemented, in order to guarantee that no unexpected problems may occur.

With all this methods implemented, this `datastore` should also run `interface-datastore` tests against this, like [here](https://github.com/ipfs/js-datastore-fs/blob/master/test/index.spec.js#L130-L155).