ProcStats : Use mutex lock to make fetch() thread safe.

Whilst investigating a possible memory issue using the Memory Auditor, I discovered the information printed was garbled if multiple threads where used.

The issue was the ProcStats::fetch method which was not thread safe. This MR fixes that with a simple mutex lock.

I also did a drive-by cleanup and code modernisation.

FYI @clemenci

GaudiAud/src/ProcStats.cpp

@@ -241,10 +241,8 @@ struct linux_proc {
 #endif // __linux__ or __APPLE__
 
 ProcStats* ProcStats::instance() {
-  static std::once_flag             alloc_instance_once;
-  static std::unique_ptr<ProcStats> inst;
-  std::call_once( alloc_instance_once, []() { inst = std::make_unique<ProcStats>(); } );
-  return inst.get();
+  static ProcStats inst{};
+  return &inst;
 }
 
 ProcStats::ProcStats() {