Revise how downloaded questionnaire responses are treated.

[mbastian93]

Currently, when executing the downloader script, all available questionnaire responses that were successfully downloaded and decrypted are deleted from the queue. For several reasons this is or might be problematic - or at least unwanted.
Admittedly, the 'delete' step in the downloader script could easily removed but that just leads to the problem, but in that way consecutive executions of the script lead to redundant downloads/decryptions.

From my point of view the following things should be implemented:

• update the queue table and add columns that indicate, whether the respective response object has been downloaded and decrypted, as well as timestamp, when the last download occurred
• update the download api to allow download of
  • entries that haven't been downloaded yet (basically, whats currently implemented)
  • all entries
  • downloaded and encrypted entries
  • (optional) entries identified by timestamp (referring to [last?] download) (maybe even range)

Consequently, the downloader script would need to be updated so that the usage of the api can be configured (e.g. with environment variables)

[mbastian93]

This issue will be closed as #57 at least replaced the download step. Some further work might be done to extend the api in order to allow parameterized queries for questionnaire response object in the queue, but that will be addressed once the details concerning the interface to the ORSC-standard have been worked out.