common/OpTracker: refactor check_ops_in_flight()

common/OpTracker: refactor check_ops_in_flight()

* add a function visit_ops_in_flight() so it accepts a std::function,
  which can be applied to all ops in flight. and is used to count
  slow ops. the # of slow ops will be sent to mgr.
* add a function with_slow_ops_in_flight(), so we can use it to
  implement command like "dump_slow_ops_in_flight". unlike the
  existing feature which dumps slow ops to clog periodically, it
  will dump the slow ops on demand. and backoff the warning
  interval only when the slow ops are dumped.

Signed-off-by: Kefu Chai <kchai@redhat.com>