- Renamed security_context -> security_connector.
- Credentials are now responsible for creating their own connectors.

- xray was useful during dev testing but is neither a dev or runtime dep
- its presence causes issues for libraries that handle signals themselves

Also
- updates grpc.gemspec with requirements

- allow BadStatus to contain metadata that's populated by keyword args
- on servers, convert raised BadStatus metadata to trailers
- on clients, convert trailers to BadStatus metadata when raising BadStatus

- adds signal handling to RpcServer
- ensures that very little behaviour occurs in the signal handling context
- adds #run_till_terminated that launches a server in a thread and then handles
  signals that can be used to stop it.

They don't need to be extern

Prevent symbol cache variables from being instantiated multipe times.

They do not need to be extern.