MXS-2414: Prototype connection attempt throttling

The RateLimit class stores authentication failure data mapped by the
client IP addresses. The authentication failures are limited
per thread. The limits are still hard-coded and at least the number of
failures should be made configurable.

The simplest, most maintainable and acceptably efficient implementation
for DDoS protection is a thread-local unordered_map. The unwanted
side-effect of "scaling" of the number of allowed authentication failures
is unlikely to be problematic in most use-cases.

As the blocking of a host is only temporary, the behavior differs from the
one in the MariaDB server. This allows the number of failures to be set to
a much lower value negating some of the problems caused by the relatively
simple implementation.

include/maxscale/listener.hh

@@ -186,6 +186,16 @@ public:
         return m_type;
     }
 
+    /**
+     * Mark authentication as failed
+     *
+     * This updates the number of failures that have occurred from this host. If the number of authentications
+     * exceeds a certain value, any attempts to connect from the remote in quesion will be rejected.
+     *
+     * @param remote The address where the connection originated
+     */
+    void mark_auth_as_failed(const std::string& remote);
+
     // Functions that are temporarily public
     bool          create_listener_config(const char* filename);
     struct users* users() const;