issue#516 modify pool scrub functionality

remove automatic polling mechanism for pool scrub.
Instead provide the scrub status information and
ability to refresh scrub status if the scrub is running.
If the scrub is not not running then provide the ability
to start a new scrub.
Merge branch 'issue#516_pool_scrub_error' of https://github.com/kamal-gade/rockstor-core into kamal-gade-issue#516_pool_scrub_error
update scrub model with new attributes. #516
Refactor/update scrub status. #516
Support force swtich for scrub. #516
Merge git://github.com/schakrava/rockstor-core into issue#516_pool_scrub_error
Merge branch 'issue#516_pool_scrub_error' of git://github.com/kamal-gade/rockstor-core into issue#516_pool_scrub_error
issue#516 modify pool scrub functionality

Display list of pool scrubs and allow option to start a
new scrub and also provide an option to force the start
of new scrub
Add service toggle on the scheduled tasks screen. #516
issue#516 modify pool scrub functionality

Display list of pool scrubs and allow option to start a
new scrub and also provide an option to force the start
of new scrub
Merge branch 'issue#516_pool_scrub_error' of https://github.com/kamal-gade/rockstor-core into kamal-gade-issue#516_pool_scrub_error
Merge branch 'master' into kamal-gade-issue#516_pool_scrub_error
fork a process and run pool scrub in foreground, makes detaching easy. #516
update status also on get. #516
updated with some ui changes
update messaging on pool detail view. #516
Simplify and refactor task dispatcher for scrub task. #516
update messaging on task history view. #516
re-add snapshot support to task scheduler(post refactor backporting). Fixes #516
remove deprecated task worker. #516