interest: deprecate selectors

NDN Packet Format v0.3 replaces Selectors with CanBePrefix
and MustBeFresh elements.
This commit deprecates getter/setter for Selectors in Interest
class. Getter/setter for CanBePrefix and MustBeFresh are mapped
to the closest v0.2 semantics and encoded as selectors.

refs #4527

Change-Id: Ie3d58d3d73f3bf1d8db3327943e6850a867465b0
diff --git a/src/selectors.hpp b/src/selectors.hpp
index 5a80185..d25c980 100644
--- a/src/selectors.hpp
+++ b/src/selectors.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2017 Regents of the University of California.
+ * Copyright (c) 2013-2018 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -126,7 +126,7 @@
   Selectors&
   setChildSelector(int childSelector);
 
-  int
+  bool
   getMustBeFresh() const
   {
     return m_mustBeFresh;