refactor: add most of src/util to iwyu

These files change infrequently, and not much header shuffling is required.

We don't add everything in src/util/ yet, because IWYU makes some
dubious suggestions, which I'm going to follow up with upstream.

src/util/thread.cpp

@@ -9,6 +9,7 @@
 #include <util/threadnames.h>
 
 #include <exception>
+#include <functional>
 
 void util::TraceThread(const char* thread_name, std::function<void()> thread_func)
 {