helper: Experimental extension of ndn::StackHelper to enable customization of NetDeviceFace (e.g., creating of custom faces based on different NetDevice's)
model: Fixing bug with hop count tag related to caching of unsolicited data
model: (Re-)Implementing ability to keep PIT records after Interest is getting satisfied

Default behavior remained the same.

As part of this commit, a small extension (and bug fix) in trie data
structure, allowing search with predicates (e.g., used to lookup a
non-empty PIT entry during Lookup on ContentObject header).