mon: OSDMonitor: allow (un)setting 'hashpspool' flag via 'osd pool set'

This PR backports to Dumpling the monitor command for setting and unsetting "hashpspool" on a per-pool basis. This is necessary:
1) We already have hashpspool functionality
1b) for some portion of time it was enabled by default (now it is disabled by default).
2) Because hashpspool is controlled via a flag in the osdmap, there is no way to manipulate it without a monitor command
3) The hashpspool functionality is useful for clusters which are running Dumpling.

So backport the relevant patches to change it. Unfortunately they had to go through quite a bit of change: Dumpling did not have all the generic "pool set" infrastructure we do in current master, so I had to special-case it. Dumpling uses integers rather than strings for the parameters, so backporting required changing that syntax. I'm not sure if there's documentation we want to provide on that.