Tidy up includes:

* Replace boost with std equivalents:
  - bind, ref, cref, function, placeholders
* More "include what you use"
* Remove unnecessary includes

beast/module/core/threads/CriticalSection.h

@@ -25,6 +25,7 @@
 #define BEAST_CRITICALSECTION_H_INCLUDED
 
 #include <beast/module/core/threads/ScopedLock.h>
+#include <cstdint>
 
 namespace beast {