face: Transport::canChangePersistencyTo

Feasibility of transport persistency change can be checked with
Transport::canChangePersistencyTo before executing the actual change.
This enables management to verify persistency and other parameters
in a faces/update command before applying the updates.

Change-Id: Ia283f0daf678f47aad7b78b7e06dee4827f57cab
refs: #3232
diff --git a/daemon/face/unicast-udp-transport.hpp b/daemon/face/unicast-udp-transport.hpp
index 80cd70b..6136746 100644
--- a/daemon/face/unicast-udp-transport.hpp
+++ b/daemon/face/unicast-udp-transport.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2014-2016,  Regents of the University of California,
+ * Copyright (c) 2014-2017,  Regents of the University of California,
  *                           Arizona Board of Regents,
  *                           Colorado State University,
  *                           University Pierre & Marie Curie, Sorbonne University,
@@ -43,8 +43,11 @@
                       time::nanoseconds idleTimeout);
 
 protected:
-  virtual void
-  beforeChangePersistency(ndn::nfd::FacePersistency newPersistency) final;
+  bool
+  canChangePersistencyToImpl(ndn::nfd::FacePersistency newPersistency) const final;
+
+  void
+  afterChangePersistency(ndn::nfd::FacePersistency oldPersistency) final;
 
 private:
   void