Allow proper signedPreKey rotation

[vanitasvitae]

Background: The client should generate and publish a new singedPreKey in their bundle every now and then (7-14 days). The old signedPreKey must be kept for ~a month in order to decrypt delayed preKeyMessages that were encrypted using the old signedPreKey. In order to strengthen forward secrecy, the old key should be deleted after a month though.

Possible solutions:

• Rotate keys every n days and ensure that maximal floor(28/n) keys are kept in storage.
• Store a creation date for every key and rotate/delete when keys are too old.

Manual rotation of the signedPreKey is already possible

[vanitasvitae]

Keys are renewed every week now.

[vanitasvitae]

Done. Old keys are now also removed.