osd: Retain overridden mClock recovery settings across osd restarts

osd: Retain overridden mClock recovery settings across osd restarts

Fix an issue where an overridden mClock recovery setting (set prior to
an osd restart) could be lost after an osd restart.

For e.g., consider that prior to an osd restart, the option
'osd_max_backfill' was successfully set to a value different from the
mClock default. If the osd was restarted for some reason, the
boot-up sequence was incorrectly resetting the backfill value to the
mclock default within the async local/remote reservers. This fix
ensures that no change is made if the current overriden value is
different from the mClock default.

Modify an existing standalone test to verify that the local and remote
async reservers are updated to the desired number of backfills under
normal conditions and also across osd restarts.

Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>