Implement Cayley as a QuadStore backend for Cayley

Description

This is a feature request.

The API exposed through imports.go, allows us to query Cayley in native Go code, rather than write SQL-like Sprintf commands which are then parsed back into Go as it is done in cayleygraph/go-client.

It would be nice, if the Iterator operations could be relayed to another Cayley instance over gRPC, rather than sent to a "local" (i.e. owned) backing store accessed via hidal-go.

Meta.