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
Merge branch 'master' into congestion