librbd: new alternative write-around cache layer

librbd: new alternative write-around cache layer

This is a replacement for the ObjectCacher-based writethrough/
writeback cache. It permits a configurable maximum byte size
of in-flight writes (bytes) to the OSDs where the writes are
immediately completed back to the caller. Any IO errors are
propagated on the next flush request.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>