Since alarm checks may mutate work deadlines for pollsets, the value
returned from maybe_work is meaningless. Instead, maybe pollset_work
always return 1 if maybe_work is invoked, and then redo the deadline
check _on the next call_ to pollset_work.

- introduces a #wait method on the call operation view
- invokes #wait on a Notifier that is created for all operations
- ensures the Notifier is invoked if necessary whenever a client request
  completes
- updates the interop_test to use op.wait before checking if the call
  was cancelled.

- ALPN may not be available:
  - because OpenSSL does not support it.
  - because one of the peers does not support it.
- Tested manually end to end by forcing TSI_OPENSSL_ALPN_SUPPORT to 0.
  Also tested if only one of the peers supports ALPN.

state (including channels)

It was still linking to my pull request's podspec.

Pull request #1577 removes the backup poller internal to completion
queues so this test needs to use extra threads to drive the work done
inside the completion queues by continuously calling each completion
queue's get method.

can be removed from C++ lock.

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

Through the metadata's user data mechanism.

Fixes two warnings in log_win32.c under msvc2015
log_win32.c(109): warning C4013: 'gpr_strdup' undefined; assuming extern returning int
log_win32.c(109): warning C4047: 'return': 'char *' differs in levels of indirection from 'int'

and corrects a typo in an error message