GEOMESA-331 - Attach IndexSchema to SFTs

* Refactored IndexSchemas to be on the SFTs rather than on accessing GeoMesa DataStores.
* Note that in the tests, for MockAccumulo, max shards is 0.
i think warn is appropriate - this would only happen if they mis-configured something, and as a result they're not going to end up with the behavior they expect.