Allow scrubbing while thrashing

Added ability to implement scrubbing while thrashing
(scrub_interval in config can be set to an interval
similar to how clean_interval is set).  Defaults to 0,
which indicates that no scrubbing will take place.
Add scrub_interval description to thrashosds docstring.

Fixes: 7199
Signed-off-by: Warren Usui <warren.usui@inktank.com>
Merge pull request #244 from ceph/wip-7199-wusui

Wip 7199 wusui