mon: warn in newfs if crash_replay_interval=0

This is the setting we would apply to data pools
created automatically, so notify the user if they're
failing to use it for data pools they have created
by hand.

Signed-off-by: John Spray <john.spray@inktank.com>
mon: enable deleting pools when FS disabled

Signed-off-by: John Spray <john.spray@inktank.com>
mon: stop MDSMonitor when FS disabled

Where 'stopped' means ignoring beacons and
not performing 'tick' activities.

Signed-off-by: John Spray <john.spray@inktank.com>
mon: add `ceph mds rmfs`

Sets enabled=false on the mds map so that an
administrator can remove the filesystem pools
if he wishes.

Signed-off-by: John Spray <john.spray@inktank.com>
mon: refactor MDSMonitor cmd handler

MDS commands are now divided into two handler functions,
one for create/remove of the filesystem, and one for
operations on the filesystem itself.

Signed-off-by: John Spray <john.spray@inktank.com>
mon: forbid 'mds rmfs' while daemons are running

This avoids the need to do anything clever about kicking
out MDS daemons during a rmfs: we push that responsibility
onto the admin to run 'mds stop' or somestuch first.

Signed-off-by: John Spray <john.spray@inktank.com>