MXS-860: Detect whether replication is configured

The `detect_stale_slave` functionality used to only work when MaxScale had
the knowledge that a master server has existed and that replication was
working at some point in time. This might be a "safe" way to do it in
regards to staleness of the data but in practice it is preferrable to
always allow slave to be used for reads.

This change adds the missing functionality to the monitor by assigning
slave status to all servers which are configured as replication slaves
when no master can be found.

The new member variable that was added to the SERVER should be removed in
2.1 where the server_info offers the same functionalty without "polluting"
the SERVER type.

Documentation/Monitors/MySQL-Monitor.md

@@ -78,10 +78,6 @@ With this parameter, slaves that have lost their master but have been slaves of
 a master server can retain their slave status even without a master. This means
 that when a slave loses its master, it can still be used for reads.
 
-If MaxScale loses the connection to the slave, the slave will lose the stale
-slave state because MaxScale doesn't know if the slave has had recent contact
-with the master server.
-
 If this feature is disabled, a server is considered a valid slave if and only if
 it has a running master server monitored by this monitor.
 

Documentation/Release-Notes/MaxScale-2.0.1-Release-Notes.md

@@ -120,6 +120,7 @@ Please consult
 * [MXS-845](https://jira.mariadb.org/browse/MXS-845): "Server down" event is re-triggered after maintenance mode is repeated
 * [MXS-842](https://jira.mariadb.org/browse/MXS-842): Unexpected / undocumented behaviour when multiple available masters from mmmon monitor
 * [MXS-846](https://jira.mariadb.org/browse/MXS-846): MMMon: Maintenance mode on slave logs error message every second
+* [MXS-860](https://jira.mariadb.org/browse/MXS-860): I want to access the web site if master server is down.
 
 ## Known Issues and Limitations