Config changes need a client restart, possible re-registration

[Andreas Hasenack]

The client needs to be restarted after configuration changed, and, depending on the key that changed, even re-registered.

In common.py, update_client_configuration():
- if there were changes, the client needs to be restarted (or restart for all changes)
- if these keys changed, the client also needs to register again:
  - ping_url
  - url
  - account_name

There could be others, but I'm not sure what the behavior would be, or in which scenarios they are changed, like "data_path". I think the above list is a good start.

Strictly speaking, a change in ping_url doesn't neeed a re-registration, but I can't see a situation where it will be changed alone without also changing url.