Improve charset selection

The charset sent in the handshake is now done with the following
priorities:

* First Master server
* Last Slave server
* First Running server or Down server whose charset is known

The change is that server in the Down state to which we've successfully
connected to can also be used as the charset source. This, in addition
with an "empty" default charset, helps avoid the use of the default latin1
charset unless absolutely necessary.

include/maxscale/server.hh

@@ -181,8 +181,10 @@ public:
                                      * routing sessions. */
 
     // Base variables
-    bool    is_active = false;          /**< Server is active and has not been "destroyed" */
-    uint8_t charset = DEFAULT_CHARSET;  /**< Character set. Read from backend and sent to client. */
+    bool    is_active = false;  /**< Server is active and has not been "destroyed" */
+    uint8_t charset = 0;        /**< Character set. Read from backend and sent to client. As no character set
+                                 * has the numeric value of 0, it can be used to detect servers we haven't
+                                 * connected to. */
 
     // Statistics and events
     ConnStats stats;            /**< The server statistics, e.g. number of connections */
@@ -535,7 +537,6 @@ protected:
     }
 
 private:
-    static const int   DEFAULT_CHARSET = 0x08;      /**< The latin1 charset */
     maxbase::EMAverage m_response_time;             /**< Response time calculations for this server */
     std::mutex         m_average_write_mutex;       /**< Protects response time from concurrent writing */
     mxs::SSLProvider   m_ssl_provider;