nlsr: Add support for sources to NamePrefixList

Added support for string sources to NamePrefixList entries.  Required was
some tweaking of the API, and refactoring usage around the codebase.

Change-Id: I44813e024a88dc1f591f427b0fad568a7d5353ab
refs: #4177
diff --git a/tests/publisher/publisher-fixture.hpp b/tests/publisher/publisher-fixture.hpp
index c52f378..9156c7b 100644
--- a/tests/publisher/publisher-fixture.hpp
+++ b/tests/publisher/publisher-fixture.hpp
@@ -140,7 +140,7 @@
 
     std::list<ndn::Name>::const_iterator it = tlvLsa.getNames().begin();
 
-    for (const ndn::Name& name : lsa.getNpl().getNameList()) {
+    for (const ndn::Name& name : lsa.getNpl().getNames()) {
       BOOST_CHECK_EQUAL(*it, name);
       ++it;
     }