mon/OSDMonitor: limit number of concurrently creating pgs

mon/OSDMonitor: limit number of concurrently creating pgs

There is overhead for PGs we are creating because the mon has to track
which OSD each one current maps to.  This can be problematic on a very
large cluster.  Limit the overhead by setting a cap on the number of PGs
we are creating at once; leave the rest in a persistent queue.

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