OSD: resurrect a parent if it splits into the pg we want to create

When attempting to create a new pg object in response to a
peering message, there are 3 cases:
1) That pg is currently being deleted.  In this case, we
cancel the deletion and resurrect the pg at the epoch at
which it had been deleted.
2) A pg is being deleted which would have split into the
pg we want to create had it not been deleted.  In that case,
we resurrect that pg at the map at which it had been deleted
and let the request wait on the impending split.
3) Neither that pg nor a parent can be resurrected.  In this
case, we create a new pg at the map epoch of the peering
request.

Fixes: #5154
Signed-off-by: Samuel Just <sam.just@inktank.com>