The clients now block until the channel is in the READY state. This
fixes some test flakiness issues we have had.

Add response parameters to custom_metadata streaming request for Node and PHP
clients.

The Java server does not respond with separate initial and trailing
metadata when there is no response data - it is only emiting the
requested trailing metadata. Adding the response parameters to the test
(in accordance with the specification) avoids this, but I will open a
separate issue to investigate the Java behavior.

It's been confusing that these tests have been called "interop" but
are not actually tests of interoperation.

- make closures know where they should be executed (eg, on a workqueue,
  or a combiner, or on an exec_ctx)
- this allows removal of a large number of trampoline functions that
  were appearing whenever we used combiners, and should allow for a much
  easier interface to combiner locks

- Initializes grpc correctly in the JWT utils.
- Make the email mapping work with the new service accounts produced by
  Google IAM.
- Adding check for email issuers where the issuer has to be the subject as well.
- Implementing portable version of memrchr.

Added tests for detecting invocation time defects arising out of the
runtime.