MXS-2141: Retry query on master if it times out on slave

With causal_reads enabled, the query would return with an error if the
slave was not able to catch up to the master fast enough. By automatically
retrying the query on the master, we're guaranteed that a valid result is
always returned to the client.

Documentation/Routers/ReadWriteSplit.md

@@ -477,11 +477,11 @@ SELECT * FROM test.t1 WHERE id = 1;
 The `SET` command will synchronize the slave to a certain logical point in
 the replication stream (see
 [MASTER_GTID_WAIT](https://mariadb.com/kb/en/library/master_gtid_wait/)
-for more details). If the slave has not caught up to the master within the
-configured time, an error will be returned. To the client side
-application, this will appear as an error on the statement that they were
-performing. This is caused by the fact that the synchronization command is
-executed with the original command as a multi-statement command.
+for more details).
+
+If the slave has not caught up to the master within the configured time, it will
+be retried on the master. In MaxScale 2.3.0 an error was returned to the client
+when the slave timed out.
 
 ### `causal_reads_timeout`