mon: do not allow unreasonable PG splitting
We've had two different kinds of issues with PG splits:
1) the incorrectly-interpreted PG values from the Dumpling->Emperor upgrade (with a Dumpling leader) causing creation of millions of PGs,
2) Users splitting by a 255x multiple within a pool.
This causes issues with the OSD of various kinds; the first one for obvious reasons and the second one because the OSD internals are simply not designed to handle a sufficiently-large number of multiples in one go. In particular, in reproducing (2) locally I saw it taking 4 minutes to process the splits, during which time the internal heartbeating failed while waiting for stuff to get into the queue (which was at the default 50-op limit). To deal with both issues, have the monitors specify a maximum PG split ratio and enforce it.