Tls session, keep only expiry date

[icing]

Instead of "time received" and "lifetime seconds", keep only the expiry date for TLS session tickets.

Rationale:

• We do not want to export the received time, since that allows easier tracking of when connections have been made.
• We do not want to export the lifetime, since that allows easier identification of (anonymised) session tickets from a particular server. Server will have different lifetimes in their configuration and persisting that time would allow identifying tickets from, say, facebook vs. wikipedia or furries.com.

Of course, once the host for an entry is found out (maybe by brute forcing), the connection time could still be calculated from the lifetime that host is configured with. But there is no helping that.