mon/OSDMonitor: require force argument to split a cache pool

There are several perils when splitting a cache pool:

 - split invalidstes pg stats, which disables the agent
 - a scrub must be manually triggered post-split to rebuild stats
 - the pool may fill the OSDs during that period.
 - or, the pool may end up beyond the 'full' mark and once scrub does
   complete and the agent activate we may block IO for a long time while
   we catch up with flush/evict

Make it a bit harder for users to shoot themselves in the foot.

Fixes: #8043
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1681 from ceph/wip-8043

mon/OSDMonitor: require force argument to split a cache pool

Reviewed-by: Samuel Just <sam.just@inktank.com>