Service testing

[andrewramsay]

This is a collection of fixes for a few problems I found while doing some more testing of the service, particularly with multiple simultaneous clients connected:

• The database class now creates a cursor per-query instead of reusing the same one as that breaks when multiple threads are accessing it
• Added a test which will spawn 10 validator clients and run them all against the service
• gRPC exceptions can contain useful info about exceptions on the server side so those are now logged
• Added a couple of extra checks on the run file JSON structure (+tests)
• Updated the README with some examples of different parameter combinations

This branch should merge cleanly on top of the grpc_timeout branch from #21