Refine shutdown API

include/grpc++/server.h

include/grpc/grpc.h

src/core/surface/completion_queue.c

src/core/surface/server.c

src/cpp/server/server.cc

test/core/end2end/dualstack_socket_test.c

test/core/end2end/tests/bad_hostname.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

test/core/end2end/tests/cancel_before_invoke.c

test/core/end2end/tests/cancel_in_a_vacuum.c

test/core/end2end/tests/census_simple_request.c

test/core/end2end/tests/disappearing_server.c

...end/tests/early_server_shutdown_finishes_inflight_calls.c

.../core/end2end/tests/early_server_shutdown_finishes_tags.c

test/core/end2end/tests/empty_batch.c

test/core/end2end/tests/graceful_server_shutdown.c

test/core/end2end/tests/invoke_large_request.c

test/core/end2end/tests/max_concurrent_streams.c