Migrate low-level RPC API from Py4J to GRPC
- SwhGraphProperties: remove useless IOExceptions
- SwhUnidirectionalGraph: make constructor with properties public
- GRPC: initial commit with protobuf + java server
- Add CheckSwhid() and Stats() RPC methods
- RPC: temporarily use loadLabelled() because loadLabelledMapped() doesn't work yet
- Java: add proto/ dir symlink
- Python: migrate HTTP API to GRPC
- Java tests: migrate to GRPC
- Remove old Java HTTP server, replaced by GRPC
- NodeIdMap: use new *MappedBigList classes, remove deprecated classes and MapFile
- Remove now useless backend.py
- Remove now useless dot.py
- Reorganize Python files (RPC/HTTP server distinction)
Migrated from D7890 (view on Phabricator)