model: Fixing limit borrowing bug in PerOutFaceLimit and PerFibLimit

When the application sends an Interest, router will borrow limit after
forwarding the Interest. If Data gets dropped in the middle, the
application will retransmit the Interest, and router will borrow another
limit. When Data comes back, limit is only returned once. This may cause
the limit to be wasted.

http://redmine.named-data.net/projects/ndnsim refs #1001
GitHub Issues refs #37
model: Change default behavior to cache unsolicited data from local applications

In many cases, when a real NDN application is written, it is desirable
to be able to push content to the local cache (e.g., "pre-publish").

http://redmine.named-data.net/projects/ndnsim refs #1002
utils/trie: Simplify return type of get_frequency() in lfu-policy