MXS-1367: Add timeouts for retried queries

The total timeout for the retrying of interrupted queries can now be
configured with the `query_retry_timeout` parameter. It controls the total
timeout in seconds that the query can take.

The actual connection, read and write timeouts of the connector aren't a
good configuration value to use for abstracted queries as the time that it
takes to execute a query can be composed of both connections, reads and
writes. This is caused by the usage of MYSQL_OPT_RECONNECT that hides the
fact that the connector reconnects to the server when a query is
attempted.

Documentation/Release-Notes/MaxScale-2.1.10-Release-Notes.md

@@ -30,6 +30,8 @@ The internal SQL queries that MaxScale executes to load database users as well
 as monitor the database itself can now be automatically retried if they are
 interrupted. The new global parameter, `query_retries` controls the number of
 retry attempts each query will receive if it fails due to a network problem.
+The `query_retry_timeout` global parameter controls the total timeout for the
+retries.
 
 To enable this functionality, add `query_retries=<number-of-retries>` under the
 `[maxscale]` section where _<number-of-retries>_ is a positive integer.