swhgraph: use grpc API
replace the (deprecated) HTTP RCP API to access the swh-graph service, in favor of the grpc server.
To be able to test the (now) grpc-based ArchiveGraph, compressed graph datasets for all 3 common datasets (cmdbts2, out-of-order and with-merges) have been generated and included in this revision.
Depends on !145 (closed)
Migrated from D8362 (view on Phabricator)