introducer.client: use integer seqnums, not time-based. Closes #1767.

introducer.client: use integer seqnums, not time-based. Closes #1767.

This stores the sequence number in BASEDIR/announcement-seqnum, and
increments it each time any service is published (every service
announcement is regenerated with the new sequence number). As everyone
knows, time is an illusion, and occasionally goes backwards, so a
counter is generally safer (and reveals less information about the
node).

Later, we'll improve the introducer client to tolerate rollbacks (where,
perhaps due to a VM being restarted from an earlier checkpoint, the
stored sequence number reverts to an earlier version).