This will avoid a name clash when retransmission is implemented.

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.

This allows different plugins to have different dependencies without
cluttering the plugin factory API.