Save a bunch of copies in the trace thread (#7392)

Currently, a std::string is copied unnecessarily for every key and value
in a trace event.

This actually showed up in a jemalloc heap profile while I was
investigating something unrelated. I was surprised to see it since these
allocations should have a very short lifetime.

flow/Trace.cpp

@@ -254,7 +254,7 @@ public:
 			double getTimeEstimate() const override { return .001; }
 		};
 		void action(WriteBuffer& a) {
-			for (auto event : a.events) {
+			for (const auto& event : a.events) {
 				event.validateFormat();
 				logWriter->write(formatter->formatEvent(event));
 			}