- provides a noop logger unless the user explicit adds a logging method
  to the GRPC namespace

- adds the code-generated health service classes to the pb along with
  a README explaining how to regenerate the generated code

- adds an implementation of the Health Checker Service along with unit
  tests and an integration test

Also:
- adds a pb folder
  : in a follow-up PR, all ruby pbs + generated code will be moved to it

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

The C core uses completion_queue_next as a directive that progress should be made on requests bound to that cq.
If nobody is polling for completion, then a deadlock can occur (client needs to do work, but we're blocked waiting for the server).
There is no scope for this to occur using the idiomatic layers (as far as I can tell), but these low level tests need to be massaged.

- 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