This replaces all previous code with a new approach for the provenance
service.

This will propose an RPC (like other main SWH services) allowing to get
provenance informations from several possible backends. Currently, 2
backends are considered:
- a swh-graph one, using the swh-graph service to answer the provenance
  requests,
- a postgresql one using a provenance index stored in a postgresql
  database, built from the provenance dataset exported from swh-graph
  using dedicated code.