Rewrite mxs564_big_dump

Streamlined the test to perform as much testing as fast as possible. The
gradual ramp up did not provide any concrete benefits compared to testing
everything at once.

Replaced structures with C++11 alternatives where possible and removed
unused, redundant or dead code.

maxscale-system-test/testconnections.cpp

@@ -1688,6 +1688,8 @@ void *log_copy_thread( void *ptr )
         Test->tprintf("\n **** Copying all logs *** \n");
         Test->copy_all_logs_periodic();
     }
+
+    return NULL;
 }
 
 int TestConnections::insert_select(int m, int N)