This will avoid a name clash when retransmission is implemented.

This removes the need to send a start time for each subscription; new
subscribers will receive old messages if and only if there's bandwidth.

The structure of the reader now resembles that of the writer, which
should make it easier to spot discrepancies.

The old logic for selecting when to send subscription and transport
updates has been removed and not yet replaced. Subscription times have
been removed from subscription updates. The database expiry time has
been remove from subscription updates and will later get its own update
packet. Transport updates have been broken up into one update per
transport. Acks for subscription and transport updates have been added.