lifecycle/shutdown

[raboof]

Currently, NextcloudConnector has a shutdown method that calls the static ConnectorCommon.shutdown();

That seem a bit weird if you have multiple NextcloudConnector instances that could be around at the same time: in that case you have to make sure you delay shutting down until all connectors have completed.

I guess we could add a helper that reference-counts how many NextcloudConnectors have been instantiated, and shut down ConnectorCommon only when the last one is shut down?