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.

server/modules/protocol/MySQL/mariadbclient/mysql_client.cc

@@ -818,6 +818,8 @@ static int gw_read_do_authentication(DCB* dcb, GWBUF* read_buffer, int nbytes_re
     {
         protocol->protocol_auth_state = MXS_AUTH_STATE_FAILED;
         mysql_client_auth_error_handling(dcb, auth_val, next_sequence);
+        mxb_assert(dcb->session->listener);
+        dcb->session->listener->mark_auth_as_failed(dcb->remote);
         /**
          * Close DCB and which will release MYSQL_session
          */