osd: throttle snap trimmming with simple delay

This is not particularly smart, but it is *a* knob that lets you make
the snap trimmer slow down.  It's a flow and a simple delay, so it is
adjustable at runtime.  Default is 0 (no change in behavior).

Partial solution for #6278.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1694 from ceph/wip-throttle-snap-master

osd: throttle snap trimmming with simple delay

Reviewed-by: Samuel Just <sam.just@inktank.com>