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/random.cc

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2016 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2022-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include <maxscale/random.h>
+
+#include <random>
+
+namespace
+{
+
+thread_local std::mt19937 generator(std::random_device{}());
+
+}
+
+unsigned int mxs_random(void)
+{
+    return generator();
+}