try to fix multi FaceThread loss

observation:
* no problem if ccnpingserver starts before ccnping
* only one Interest is sent to ccnpingserver,
  if ccnpingserver starts after ccnping

fix:
* do nothing in Strategy::DidAddFibEntry,
  but pending Interests won't be satisfied by new FIB entry

status:
* no problem regardless of starting order
  if there are <=4 FaceThreads