Cleanups:

* Reduce Beast dependencies
* Remove unnecessary includes
* Don't use deprecated bassert macros
* Don't use beast::String in Json::Value

src/ripple/server/impl/ServerImpl.h

@@ -45,7 +45,6 @@ class Door;
 struct Stat
 {
     std::size_t id;
-    std::string when;
     std::chrono::seconds elapsed;
     int requests;
     std::size_t bytes_in;