librbd ioengine support

Initially only DDIR_WRITE/DDIR_READ and DDIR_SYNC are supported.
All other io_u->ddir get silently ignored.

A known issue requires to call the rbd engine always with --invalidate=0

Signed-off-by: Daniel Gollub <d.gollub@telekom.de>
Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
fio.1: add rbd options to man page

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>

(cherry picked from commit 6e44a82a724b7ad0a559c22eec694e8afa7d2a20
git@github.com:dalgaaf/fio.git)
add example job file for the RBD engine

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>