Reject old timestamps when deriving rotation mode keys

This branch checks that the timestamp is no more than one year old no earlier than 1 Jan 2021 before trying to derive rotation mode transport keys. This avoids spending an excessive amount of time deriving keys if the timestamp is very old.

Callers handle an invalid timestamp in the same way as an invalid public key (for example, aborting the session in the case of the introduction client).

If there's a gap of more than a year between the first introducee accepting an introduction request and the second introducee accepting it, the session will be aborted. I think this is a less serious problem than blocking the DB for a long time while deriving keys if one of the introducees sends an invalid timestamp. We can adjust the tradeoff between these two problems by adjusting the maximum timestamp age.

Closes #2079 (closed).