Refactor pre-switchover, add similar checks as in failover

Now detects some erroneous situations before starting switchover.
Switchover can be activated without specifying current master.
In this case, the cluster master server is selected.

server/core/monitor.cc

@@ -2427,3 +2427,16 @@ static bool journal_is_stale(MXS_MONITOR *monitor, time_t max_age)
 
     return is_stale;
 }
+
+MXS_MONITORED_SERVER* mon_get_monitored_server(MXS_MONITOR* mon, SERVER* search_server)
+{
+    ss_dassert(mon && search_server);
+    for (MXS_MONITORED_SERVER* iter = mon->monitored_servers; iter != NULL; iter = iter->next)
+    {
+        if (iter->server == search_server)
+        {
+            return iter;
+        }
+    }
+    return NULL;
+}