table: MeasurementsAccessor findLongestPrefixMatch & findExactMatch

This commit also improves unit testing of MeasurementsAccessor,
and changes signature of MeasurementsAccessor constructor.

refs #2314

Change-Id: Ie0494628c6d3faac0473bc4cfb21998f89270aaf
diff --git a/daemon/table/measurements-accessor.cpp b/daemon/table/measurements-accessor.cpp
index 81b1432..47b4fe2 100644
--- a/daemon/table/measurements-accessor.cpp
+++ b/daemon/table/measurements-accessor.cpp
@@ -1,12 +1,12 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2014,  Regents of the University of California,
- *                      Arizona Board of Regents,
- *                      Colorado State University,
- *                      University Pierre & Marie Curie, Sorbonne University,
- *                      Washington University in St. Louis,
- *                      Beijing Institute of Technology,
- *                      The University of Memphis
+ * Copyright (c) 2014-2015,  Regents of the University of California,
+ *                           Arizona Board of Regents,
+ *                           Colorado State University,
+ *                           University Pierre & Marie Curie, Sorbonne University,
+ *                           Washington University in St. Louis,
+ *                           Beijing Institute of Technology,
+ *                           The University of Memphis.
  *
  * This file is part of NFD (Named Data Networking Forwarding Daemon).
  * See AUTHORS.md for complete list of NFD authors and contributors.
@@ -30,11 +30,11 @@
 using fw::Strategy;
 
 MeasurementsAccessor::MeasurementsAccessor(Measurements& measurements,
-                                           StrategyChoice& strategyChoice,
-                                           Strategy* strategy)
+                                           const StrategyChoice& strategyChoice,
+                                           const Strategy& strategy)
   : m_measurements(measurements)
   , m_strategyChoice(strategyChoice)
-  , m_strategy(strategy)
+  , m_strategy(&strategy)
 {
 }
 
@@ -43,9 +43,9 @@
 }
 
 shared_ptr<measurements::Entry>
-MeasurementsAccessor::filter(const shared_ptr<measurements::Entry>& entry)
+MeasurementsAccessor::filter(const shared_ptr<measurements::Entry>& entry) const
 {
-  if (!static_cast<bool>(entry)) {
+  if (entry == nullptr) {
     return entry;
   }