mon: OSDMonitor: fix #6047

What seemed like a bug due to an exposed uncommitted state turned out to be that we weren't checking whether the  pools on both 'osd pool mksnap' and 'osd pool rmsnap' are in unmanaged snaps mode.

This patch series also reworks prepare_pool_op() in a way that avoids that we end up proposing the current pending state directly from the function instead of letting PaxosService handle it.