change Tracer::dumpContextForLabel into a vstring
change the parameter name to dumpContextForLabels and the internal data structure from std::vector<std::string> to std::set<std::string> to avoid the linear lookup
Merge pull request #1168 from fwyzard/Tracer_dumpContextForLabel_vstring

Multithreaded framework -- change Tracer::dumpContextForLabel into a vstring