MXS-2546 Add DNS-resolving to topology detection

When matching hostnames between MaxScale server configuration and the
SHOW SLAVE STATUS-output, use DNS-resolution if a simple string comparison
doesn't find an answer. Results of the resolution are saved to avoid
repeating the operation for the same address.

server/modules/monitor/mariadbmon/mariadbmon.hh

@@ -169,6 +169,21 @@ private:
         bool result_waiting = false;        /* Guard variable for has_result */
     };
 
+    class DNSResolver
+    {
+    public:
+        std::string resolve_server(const std::string& host);
+
+    private:
+        struct MapElement
+        {
+            std::string    address;
+            mxb::TimePoint timestamp;
+        };
+
+        std::unordered_map<std::string, MapElement> m_mapping; // hostname -> address cache
+    };
+
     ManualCommand m_manual_cmd;     /* Communicates manual commands and results */
 
     // Server containers, mostly constant.
@@ -185,6 +200,8 @@ private:
     bool m_cluster_modified = false;        /* Has a cluster operation been performed this loop? Prevents
                                              * other operations during this tick. */
 
+    DNSResolver m_resolver;                 /* DNS-resolver with cache */
+
     /* Counter for temporary automatic cluster operation disabling. */
     int cluster_operation_disable_timer = 0;