osdc/Objecter: improve pool deletion detection

Currently we can have a race like so:

 - send op in epoch X
 - osd replies
 - pool deleted in epoch X+1
 - client gets X+1, sends map epoch check
 - client gets reply
   -> fails assert that there is no map check in flight

Avoid this situation by inferring that the pool is deleted when we see
that we previously sent the request but the pool is no longer present.
Since pool ids are not reused there is no point in doing a synchronous
map check at all.

Backport: giant
Fixes: #10372
Signed-off-by: Sage Weil <sage@redhat.com>