MXS-2313: Add server ranks

The servers now accept a rank parameter that tells which servers to
prioritize.

server/core/internal/server.hh

@@ -62,6 +62,11 @@ public:
         m_settings.persistmaxtime = persistmaxtime;
     }
 
+    void set_rank(int rank)
+    {
+        m_settings.rank = rank;
+    }
+
     bool have_disk_space_limits() const override
     {
         std::lock_guard<std::mutex> guard(m_settings.lock);
@@ -117,6 +122,11 @@ public:
         return m_settings.authenticator;
     }
 
+    int rank() const override
+    {
+        return m_settings.rank;
+    }
+
     /**
      * Get a DCB from the persistent connection pool, if possible
      *
@@ -331,6 +341,8 @@ private:
         long persistpoolmax = 0;    /**< Maximum size of persistent connections pool */
         long persistmaxtime = 0;    /**< Maximum number of seconds connection can live */
 
+        int rank;   /*< The ranking of this server, used to prioritize certain servers over others */
+
         /** Disk space thresholds. Can be queried from modules at any time so access must be protected
          *  by mutex. */
         DiskSpaceLimits disk_space_limits;