mon/PGMonitor: add max pgs per osd warning

This is just like the min warning, but a max.  It should help warn users
away from having too many PGs in their cluster, which can be very
dangerous.

Signed-off-by: Sage Weil <sage@redhat.com>
mon: tighten pg vs osd warning bounds to 3x above/below 100 (target)

Previously we were warning at (effectively) 60, so this is still less
noisy than before on the min.  The max threshold is new.

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