Progress towards mdstr elimination

include/grpc/slice.h

src/core/ext/transport/chttp2/transport/chttp2_plugin.c

src/core/lib/transport/method_config.c

src/core/lib/transport/transport_op_string.c

test/core/end2end/cq_verifier.c

test/core/end2end/cq_verifier.h

test/core/end2end/end2end_test_utils.c

test/core/end2end/end2end_tests.h

test/core/end2end/tests/authority_not_supported.c

test/core/end2end/tests/bad_hostname.c

test/core/end2end/tests/binary_metadata.c

test/core/end2end/tests/call_creds.c

test/core/end2end/tests/cancel_after_accept.c

test/core/end2end/tests/cancel_after_client_done.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/cancel_with_status.c

test/core/end2end/tests/compressed_payload.c

test/core/end2end/tests/default_host.c