Compilation time improvements. (#52)

* Precompiled header for all common library headers (with cmake 3.16rc3).
* Divided cmake build into shared libraries.
* Added gold linker support.
* Separated websockets lambda expressions to an independent file.

src/util.cpp

@@ -1,8 +1,4 @@
-#include <string>
-#include <sodium.h>
-#include <sstream>
-#include <chrono>
-#include <rapidjson/document.h>
+#include "pchheader.hpp"
 
 namespace util
 {