- 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.

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

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

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

Which includes its propagation to grpc_ioreq and validation mechanisms for checking that only known bits are set ot in).
Also added an internal flag (GRPC_WRITE_INTERNAL_COMPRESS) mask for its use in signaling compressed messages.