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/native/linux_Threads.cpp

@@ -21,6 +21,9 @@
 */
 //==============================================================================
 
+namespace beast
+{
+
 /*
     Note that a lot of methods that you'd expect to find in this file actually
     live in beast_posix_SharedCode.h!
@@ -78,3 +81,5 @@ static void swapUserAndEffectiveUser()
 
 void Process::raisePrivilege()  { if (geteuid() != 0 && getuid() == 0) swapUserAndEffectiveUser(); }
 void Process::lowerPrivilege()  { if (geteuid() == 0 && getuid() != 0) swapUserAndEffectiveUser(); }
+
+}  // namespace beast