Improve handling of post-load background jobs.

Background tasks (source location zones sorting, reconstruction of
memory plot) are now started only after trace loading is finished.
Multithreaded sorting was previously impacting trace load times.

Only one thread is used to perform both jobs, one after another.

server/TracyWorker.hpp

@@ -416,7 +416,7 @@ private:
     std::atomic<bool> m_hasData;
     std::atomic<bool> m_shutdown = { false };
 
-    std::thread m_threadMemory, m_threadZones;
+    std::thread m_threadBackground;
 
     int64_t m_delay;
     int64_t m_resolution;