common/ceph_time: add operator<< for signedspan

common/ceph_time: add operator<< for signedspan

* templatize operator<<(ostream&, duration<>), so it works for more
  duration<> classes with minimal efforts -- we just need to explicitly
  instantiate these template operators
* explicitly instantiate operator<< for timespan, signedspan, seconds
  and milliseconds. they are most likely to be used in Ceph. we can add
  more of them when necessary.

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