utils: (Re-)enabling printing "dropped" Interests/Data seen by ndn::ForwardingStrategy
model: Changing ForwardingStrategy trace output logic

As of now, ForwardingStrategy will report that packet has been
forwarded (OutData/OutInterest trace) any time packet was send out to the face, regardless
the actual status of a particular send (i.e., will report as send even if queue has
been overflown for a particular face).

Additional trace (DropData/DropInterest) will report all instances of
unsuccessful transmission of Data or Interest, including cases of queue
overflowing on faces, Interest collapsing, Interest limits drops, and other
level-3 "drops".
In simple scenarios, DropData+DropInterest should equal to TxQueue drops.
utils: Extending l2-rate-tracer to include raw packet counts

This is similar to the existing ndn-l3-rate-tracer