Mirror the contact's changes to the self-destruct timer

This branch records the self-destruct timer from incoming messages and updates the local timer to match the remote timer when appropriate:

• If the incoming message has an earlier or equal timestamp to the latest message sent or received so far, we don't update our timer
• If we haven't changed the local timer since sending our last message, we mirror the remote timer
• If we have changed the local timer since sending our last message, we only mirror the remote timer if it has changed; messages that continue to use the old timer don't overwrite our local change

Closes #1832 (closed).