cta/CTA#130 Helgrind gets spurious when using `std::mutex` in the catalogue.

With respect to the mediachanger::ZmqContextSingleton class.
Replaced std::mutex and std::lock_guard with their helgrind
friendly drop in replacements threading::Mutex and
threading::MutexLocker.

mediachanger/ZmqContextSingleton.cpp

@@ -18,6 +18,7 @@
 
 #include "ZmqContextSingleton.hpp"
 #include "common/exception/Exception.hpp"
+#include "common/threading/MutexLocker.hpp"
 #include "common/utils/utils.hpp"
 
 #include <zmq.h>
@@ -28,7 +29,7 @@ namespace mediachanger {
 //------------------------------------------------------------------------------
 // s_mutex
 //------------------------------------------------------------------------------
-std::mutex ZmqContextSingleton::s_mutex;
+threading::Mutex ZmqContextSingleton::s_mutex;
 
 //------------------------------------------------------------------------------
 // s_instance
@@ -40,7 +41,7 @@ void *ZmqContextSingleton::s_instance = nullptr;
 //------------------------------------------------------------------------------
 void *ZmqContextSingleton::instance() {
   try {
-    std::lock_guard<std::mutex> lock(s_mutex);
+    threading::MutexLocker locker(s_mutex);
 
     if(nullptr == s_instance) {
       const int sizeOfIOThreadPoolForZMQ = 1;

mediachanger/ZmqContextSingleton.hpp

@@ -18,8 +18,9 @@
 
 #pragma once
 
+#include "common/threading/Mutex.hpp"
+
 #include <memory>
-#include <mutex>
 
 namespace cta {
 namespace mediachanger {
@@ -62,7 +63,7 @@ private:
    * Mutex used to implement a critical region around the implementation of the
    * instance() method.
    */
-  static std::mutex s_mutex;
+  static threading::Mutex s_mutex;
 
   /**
    * The single instance of a ZMQ context.  A value of NULL means the ZMQ