Corrects some other issues

- fixes status return when calls fail by always returning the status

- resolves bidi_call client's failure to return an exception on bad
  status by swapping the wait for status to the read thread

  * this also improves the cancel_after_first_response test

Also

- adds a unit test that verifies that a bidi call will time out.

- timeout has been a default arg till now

- this switches it to a keyword arg with the same behavior

- in addition, it adds deadline as distinct keyword arg, allowing users
  the choice of the idiomatic(timeout) or the aligned(deadline)

+ all tests pass,
- but there are a couple of workarounds
- tests are flaky

- 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

- uses Forwardable to provide access the @call within an ActiveCall
- removes redundant methods from ActiveCall