- single global mutex (simpler, easier to make correct for now)
- properly flag kick state for workers to avoid missing wakeups (if
signal is called before wait on the cv)

core places besides iomgr

Put in blocking point annotations at places in the code where we may block for reasons other than synchronization

The GRPCCall import must be in the header because it defines a protocol
that GRPCRequestHeaders uses.

The GRPCCall.h file is already correctly imported in the
GRPCRequestHeaders.m implementation file. This header shouldn’t need to
import GRPCCall and should specify a class forward declaration for
reference.

The UnaryCall interop test is now adequately specified in
doc/interop-test-descriptions.md.