Use standard RNG

The custom random number generator can now be replaced with a C++11
RNG. This greatly improves the reliability and trustworthiness of it.

In addition to this, the conversion of the RNG to a thread-local object
removes the race condition that was present in the previous
implementation. It also theoretically improves performance by a tiny bit.

server/core/gateway.cc

@@ -53,7 +53,7 @@
 #include <maxscale/session.h>
 #include <maxscale/utils.h>
 #include <maxscale/version.h>
-#include <maxscale/random_jkiss.h>
+#include <maxscale/random.h>
 
 #include "internal/admin.hh"
 #include "internal/config.hh"
@@ -1783,9 +1783,6 @@ int main(int argc, char **argv)
         goto return_main;
     }
 
-    /** Initialize the random number generator */
-    random_jkiss_init();
-
     if (!utils_init())
     {
         const char* logerr = "Failed to initialise utility library.";