common/Throttle: update BackoffThrottle::get() to work with unsigned delay

common/Throttle: update BackoffThrottle::get() to work with unsigned delay

* in fa3acba3, we switched the delay's type from
  std::chrono::duration<double> to ceph::timespan, the former's Rep is a
  signed type, while the latter is unsigned. so we need to update the
  algorithm in get() to accomodate this change.
* use mono_clock instead of real_clock for better performance
* check for 0 timespan using count() == 0, instead of comparing it with
  a temporary object created using ceph::timespan(0) for better
  performance

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