'old' identity keys should expire once one imports a more recent one

[kotowicz]

• started conversation with new contact, sent him some messages
• came back to phone later, saw that I had to accept key of new contact
• somehow the contact had sent me 3 keys in a row
• I clicked on the most recent one
• communication worked as expected
• I made the mistake of clicking on the 1st and 2nd key exchange symbol

--> can not read messages anymore ("Bad encrypted message...") --> the wrong identity key got imported and I can not delete it --> the new contact and I can try to do 'end secure session' but a new key is never issued.

this problem could be avoided if only the most recent identity key would show up (and the old ones would have expired).

[behrmann]

I think this is somewhat related to https://github.com/WhisperSystems/TextSecure/issues/1049 It would be nice to have some way of forcing keys to be deleted, too.

[automated-signal]

GitHub Issue Cleanup: See #7598 for more information.