librbd: add an async flush

librbd: add an async flush

At this point it's a simple wrapper around the ObjectCacher or
librados.

This is needed for QEMU so that its main thread can continue while a
flush is occurring. Since this will be backported, don't update the
librbd version yet, just add a #define that QEMU and others can use to
detect the presence of aio_flush().

Refs: #3737
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
(cherry picked from commit 302b93c478b3f4bc2c82bfb08329e3c98389dd97)