librbd: add 'write_zeroes' public C/C++ API methods

librbd: add 'write_zeroes' public C/C++ API methods

Unlike the existing 'discard' option which is more of a hint to
attempt to release space, the new 'write_zeroes' APIs will ensure
that the entire provided extent is fully zeroed.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit ae6dd86b22e928dc23a385faf41cf76b0d293576)

Conflicts:
	src/librbd/api/Io.h/cc: logic exists in ImageRequestWQ files
	src/librbd/librbd.cc: trivial resolution due missing api::Io
	src/test/librbd/test_librbd.cc: trivial resolution