Rename IsThreadStringRetrieved -> IsFailureThreadStringRetrieved.

IsThreadStringRetrieved() interface suggested that it can be used for
checking any thread state, as it had an uint64_t id parameter.

The implementation ignored this parameter and checked the status of
failure thread only. This was never an issue because the code using
this function was only checking for the failure thread state.

Fixed by renaming the function to explicitly state what it does and
removing the thread id parameter.

server/TracyWorker.hpp

@@ -882,7 +882,7 @@ private:
     void HandlePostponedSamples();
     void HandlePostponedGhostZones();
 
-    bool IsThreadStringRetrieved( uint64_t id );
+    bool IsFailureThreadStringRetrieved();
     bool IsSourceLocationRetrieved( int16_t srcloc );
     bool IsCallstackRetrieved( uint32_t callstack );
     bool HasAllFailureData();