gRPC server implementation

[alrevuelta]

Implements a gRPC server that exposes a service where all crawler clients can report the peers they discover.

TODO:

• [ ] Remove hardcoded parameters in favor of cli flags
• [ ] Rethink the metadata parameters to be sent
• [ ] Place the gRPC call in the correct place
• [ ] TLS certificates or some other kind of authentication mechanism
• [ ] Implement the server logic, store peers and update them
• [ ] Export metrics to prometheus, reusing the already existing code in the armiarma-client. Note that some modifications may be needed in order to be reusable.
• [ ] Write unit tests
• [ ] Integration tests with different machines reporting to a remote endpoint

[cortze]

Closing PR due to integration of PostgreSQL as the main database in #35. All the information of the DB endpoint will be now accessible from the outside, so there is no need for a Crawler - Server communication.