ServiceInsight may issue more HTTP requests to ServiceControl than necessary

[timbussmann]

When selecting a message in the message list view, there are two "identic" HTTP requests going to the ServiceControl API to request the data.

This can be seen in the log view:

and it can also be seen on the code, that due to the call to SearchBar.NotifyPropertiesChanged(); both SequenceDiagramViewModel and MessageFlowViewModel are being triggered to create new requests.

Although the API call logic uses RestRequestWithCache, no If-None-Match headers are added to the request and the same ServiceControl controller is invoked twice.

[mauroservienti]

This will be addressed by #1208