osd: fix ACK ordering on resent ops

osd: fix ACK ordering on resent ops

The wait_for_ondisk handling fixed COMMIT ordering, but the ACKs need to
go back in the same order too.  For example:

 - op A is queued
 - client disconnects, both ACK and COMMIT replies are lost
 - client reconnects
 - op A and B are sent
 - op A is queued
 - op B is applied, ACK is sent
 - op A and B COMMITs are sent
 -> client's ack callbacks will see B and then A.

Fix this by creating a waiting_for_ack queue as well, and sending ACK
responses as needed.  Also handle the case where the ACK should be sent
immediately when the retry event is received.

Fixes: #2823
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Mike Ryan <mike.ryan@inktank.com>