mon: use TYPE_SEC for mon_scrub_interval

mon: use TYPE_SEC for mon_scrub_interval

* common/legacy_config_opts.h: drop mon_scrub_interval from legacy
  settings. as this option is not in critical path. so we can afford
  the overhead of look its value up in the map.
* common/options.cc: change mon_scrub_interval's type from TYPE_INT
  to TYPE_SECS, so we can use human friendly representations like
  "3 days" when specifying its value
* mon/Monitor.cc: adapt to the changes above.
* doc/rados: update the doc for "mon scrub interval" accordingly

Signed-off-by: Kefu Chai <kchai@redhat.com>