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.

server/include/server.h

@@ -102,6 +102,8 @@ typedef struct server
     int            depth;          /**< Replication level in the tree */
     long           slaves[MAX_NUM_SLAVES]; /**< Slaves of this node */
     bool           master_err_is_logged; /*< If node failed, this indicates whether it is logged */
+    bool           slave_configured; /**< Server is configured as a replication slave
+                                      * TODO: Remove this for 2.1 */
     DCB            *persistent;    /**< List of unused persistent connections to the server */
     SPINLOCK       persistlock;    /**< Lock for adjusting the persistent connections list */
     long           persistpoolmax; /**< Maximum size of persistent connections pool */