OpTracker: use mark_event rather than _mark_event

mark_event() interfaced changed to accept time and it is default
to 'now'. The mark_event is the wrapper function of _mark_event()
and it has the check for optracking enabled/disabled as well.
The _mark_event() is now a private function.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
OpTracker: The optracker enabled/disabled check is added

The optracker enabled/disabled check is now added in the
mark_event(const string&) interface.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
Merge pull request #2065 from somnathr/wip-sd-optracker

Pull request for optracker changes

Reviewed-by: Samuel Just <sam.just@inktank.com>