osdc/Objecter: handle reply race with pool deletion

osdc/Objecter: handle reply race with pool deletion

We need to handle this scenario:

 - send request in epoch X
 - osd replies
 - pool is deleted in epoch X+1
 - client gets map X+1, sends a map check
 - client handles reply
   -> asserts that no map checks are in flight

This isn't the best solution.  We could infer that a map check isn't needed
since the pool existed earlier and doesn't now.  But this is firefly and
the fix is no more expensive than the old assert.

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