Fix proxy, finalize API

include/grpc/grpc.h

src/core/surface/call.c

src/core/surface/completion_queue.c

test/core/end2end/fixtures/proxy.c

test/core/end2end/tests/cancel_after_accept.c

...ore/end2end/tests/cancel_after_accept_and_writes_closed.c

test/core/end2end/tests/cancel_after_invoke.c