osdc/Objecter: fix pool dne corner case

osdc/Objecter: fix pool dne corner case

We have a request that is queued before a pool exists, there is one
epoch where it exists, and then the pool disappears again.  The two maps
are processed at the same time.  For the first we set needs_resend, map to
an osd, and remove from the homeless sessin.  For the second, the pool
dne, we set osd back to -1, and send a map check request.  Finally,
handle_osd_maps scans need_resend, sees the pool dne, and removes from
need_resend.  The difference from the "usual" case is that we are neither
on the need_resend list nor on the homeless session.

Fix this by concluding immediately that the pool existed (briefly) and
then no longer exists.

Fixes: http://tracker.ceph.com/issues/19552
Signed-off-by: Sage Weil <sage@redhat.com>