librbd: simplify IO flush handling through AsyncOperation

librbd: simplify IO flush handling through AsyncOperation

Allow ImageFlushRequest to directly execute a flush call through
AsyncOperation. This will allow the flush to be directly linked
to its preceeding IOs.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 09e4127d5df1e2a79f2536dc784ec3730feea9ce)

Conflicts:
	src/librbd/ImageCtx.h/cc: dropped changes
	src/librbd/io/AsyncOperation.cc: kept original helper
	src/librbd/io/ImageRequest.cc: merged new logic in flush request
	src/test/librbd/io/test_mock_CopyupRequest.cc: DNE
	src/test/librbd/io/test_mock_ImageRequest.cc: tweaked to match new behavior