SL-17483: Make ThreadPool inherit LLInstanceTracker

(cherry picked from commit 41d6a0e222241606c317281e2f0b211e16813dd5)

indra/llcommon/threadpool.cpp

@@ -22,6 +22,7 @@
 #include "stringize.h"
 
 LL::ThreadPool::ThreadPool(const std::string& name, size_t threads, size_t capacity):
+    super(name),
     mQueue(name, capacity),
     mName("ThreadPool:" + name),
     mThreadCount(threads)

indra/llcommon/threadpool.h

@@ -22,8 +22,10 @@
 namespace LL
 {
 
-    class ThreadPool
+    class ThreadPool: public LLInstanceTracker<ThreadPool, std::string>
     {
+    private:
+        using super = LLInstanceTracker<ThreadPool, std::string>;
     public:
         /**
          * Pass ThreadPool a string name. This can be used to look up the