face: Finalizing separation between registerPrefix and setInterstFilter

New registerPrefix and setInterestFilter methods allow distinct
operations of registering with local NDN forwarder (e.g., using RIB
management protocol) and setting up application-specific OnInterest call
dispatch using InterestFilters.

Change-Id: I7e4f069da131a21574a2fd9cd20e8703c34e7008
Refs: #1275