osd/scrub: increase the default scrub load limit

osd/scrub: increase the default scrub load limit

Modifying the default value of osd_scrub_load_threshold from 0.5
to 10.0, rendering it mostly irrelevant.

That is done following a combination of reasons:
- the new mclock scheduler, which provides a better mechanism to
  control scrub load under all conditions;
- the inapplicability of the existing mechanism to Crimson; and
- users` requests.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>