MXS-1506: Allow immediate query re-routing

Now that the readwritesplit uses the same mechanism for both
retry_failed_reads and delayed query retries, the re-routing function
should accept a delay of 0 seconds. This makes the mechanism more suitable
for other uses e.g. delaying of queries in filters.

include/maxscale/session.h

@@ -633,7 +633,7 @@ session_dump_statements_t session_get_dump_statements();
  * @param session The current Session
  * @param down    The downstream component, either a filter or a router
  * @param buffer  The buffer to route
- * @param seconds Number of seconds to wait before routing the query
+ * @param seconds Number of seconds to wait before routing the query. Use 0 for immediate re-routing.
  *
  * @return True if queuing of the query was successful
  */