osd: change scrub min/max thresholds

osd: change scrub min/max thresholds

The previous 'osd scrub min interval' was mostly meaningless and useless.
Meanwhile, the 'osd scrub max interval' would only trigger a scrub if the
load was sufficiently low; if it was high, the PG might *never* scrub.

Instead, make the 'min' what the max used to be.  If it has been more than
this many seconds, and the load is low, scrub.  And add an additional
condition that if it has been more than the max threshold, scrub the PG
no matter what--regardless of the load.

Note that this does not change the default scrub interval for less-loaded
clusters, but it *does* change the meaning of existing config options.

Fixes: #3786
Signed-off-by: Sage Weil <sage@inktank.com>