Fix problems fetching performance and memory issues (#724)

* request only alert message when invoking alert.get

* fetch problem alerts on demand

* problems panel: refactor

src/datasource-zabbix/zabbix/connectors/zabbix_api/zabbixAPIConnector.js

@@ -421,7 +421,12 @@ export class ZabbixAPIConnector {
   getEventAlerts(eventids) {
     const params = {
       eventids: eventids,
-      output: 'extend',
+      output: [
+        'eventid',
+        'message',
+        'clock',
+        'error'
+      ],
       selectUsers: true,
     };