TODO:
- Documentation
- Make public
- Check I’ve implemented all NSMutDict required methods

Context management is implemented.

Stub deletion now cancels all RPCs immediately.

For a termination action to properly mark an _End as having stopped it
must clear the _End's _cycle attribute. To be able to do that the
termination action must hold a reference to the _End instance.
Migrating the _termination_action behavior that creates termination
actions into the scope of the _End instance is the best way to afford
the needed instance reference.

Also passing empty string as opposed to NULL when the method name is not
found. This is much less error-prone.

The purpose of this is to be able to install a composition policy that
describes which types are incompatible and that will be enforced during
call creds composition.

If this functionality is wanted it will be done in an additive function in
the API like :

void grpc_call_credentials_set_composite_policy(
  grpc_call_credentials_composite_policy policy);

* Reference discussion: https://github.com/grpc/grpc/pull/4108#discussion-diff-44494393

- server logger.level = WARN
- client logger.level = INFO

- truncates the server debug log entries to make that readable

This fixes the current ruby server interop test flakiness.

- The interop tests recently changed so that clients access servers in
parallel, meaning that the interop servers handle multiple rpcs at once,
which is how this bug became visible

- since this change, tests run against the ruby interop server have
failed sporadically

- the problem was that the block in #loop_handle_server_calls that is
  passed to the thread pool referenced a var in an enclosing block,
  which resulted in requests being processed by the wrong handler

- this fix ensures that the block to be run on the thread pool thread
  does not have any references to vars in the enclosing block

Fixes an omission from earlier PRs that adds support metadata.

The encoding of the issuer field in this cert is now a PRINTABLESTRING
as opposed to UTF8STRING in the previous server1.pem which was causing
the Go issue.

Fixes #4086.

If:
- one thread issues a kick forcing pollset re-evaluation
- concurrently with a second thread forcing a specific poller to be awoken
And:
- both threads kicks are processed as a single wakeup
Then:
- since we enqueue nothing to the exec_ctx in this situation, we responded to the wakeup by doing another poll until the timeout, ignoring urgent work up the stack

Fix this by flagging that a specific worker was designated to be awoken (since this is a good signal that we really really need to wake up), and use that to still re-evaluate the poll set, but with an immediate deadline so that we fall out of the poll loop as soon as possible.