sourcegraph-indexserver: add grpc server (#920)
Relates to SPLF-874
This adds a grpc server to sourcegraph-indexserver. For now it supports just one method.
The diff is quite big, so I left comments to mark the most important bits.
I used the opportunity to clean up a bit (=> hence the big diff):
- Reuse grpc logic from webserver and move those bits to "/gprc/..."
- Move "protos" inside the new "grpc" directory (=> requires changes of import statements in Sourcegraph)
- Refactor import aliases for grpc packages across the codebase
Test plan:
I tested this locally by calling the new grpc endpoint directly.