General tidy and refactoring:

* Use nullptr (C++11) instead of NULL.
* Put each file into its own namespace declaration.
* Remove "using namespace" directives and add scope qualifiers.
* Control when beast's implementation of std::equal (C++14) is used.
* Tidy up some const declarations.

Conflicts:
	src/ripple_app/shamap/SHAMapSync.cpp
	src/ripple_app/tx/TransactionEngine.cpp

modules/beast_core/diagnostic/UnitTestUtilities.cpp

@@ -17,6 +17,9 @@
 */
 //==============================================================================
 
+namespace beast
+{
+
 namespace UnitTestUtilities
 {
 
@@ -197,3 +200,5 @@ public:
 };
 
 static UnitTestUtilitiesTests unitTestUtilitiesTests;
+
+}  // namespace beast