Cleaner approach to things related to stores and queries

* got rid of the Free[Command]
* RDFStore is now a typeclass
* it is made of 3 three independent ones: GraphStore, SparqlEngine, Transactor
* there is an additional Lifecycle typeclass for start/stop operations
  * it comes with a default instance doing nothing
* syntax enhancement are made available from within the typeclass themselves
It seems so for the moment. But there should be a warning there in the documentation. The signature is misleading, and there is no corrective documentation there.