librbd: acquire exclusive-lock during copy on read

librbd: acquire exclusive-lock during copy on read

Fixes: http://tracker.ceph.com/issues/18888
Signed-off-by: Venky Shankar <vshankar@redhat.com>
(cherry picked from commit 7dba5311b12011a4a6e8564e68150e54c5af5ddd)

Conflicts:
    src/librbd/AioImageRequestWQ.h:
      - in master this file has morphed into src/librbd/io/ImageRequestWQ.h
      - kraken has AioImageRequest<ImageCtx> instead of ImageRequest<ImageCtx>
    src/librbd/image/RefreshRequest.cc:
      - rename image context element to "aio_work_queue" (from "io_work_queue")
        because kraken doesn't have de95d862f57b56738e04d77f2351622f83f17f4a
    src/test/librbd/image/test_mock_RefreshRequest.cc:
      - rename image context element to "aio_work_queue" (from "io_work_queue")
        because kraken doesn't have de95d862f57b56738e04d77f2351622f83f17f4a