mon/PGMap: clear pool sum when last pg is deleted

Use the x.0 pg as a sentinel for the existence of the pool.  Note that we
have to clean in up two paths: apply_incrmenetal (which is actually
deprecated) and the normal PGMonitor refresh.

Fixes: #7912
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1573 from ceph/wip-7912

mon/PGMap: clear pool sum when last pg is deleted

Reviewed-by: Greg Farnum <greg@inktank.com>