add support for logical replication protocol v3

[heavycrystal]

This version of the protocol allows subscribers to act on prepared transactions. Previously, the prepare part of prepared transactions was not visible to subscribers and they acted like committed/rolled back (only visible if streamed) transactions.

This protocol version adds new message types and doesn't change existing ones.

Tests added.

[jackc]

It seems reasonable to me, but I don't have much context to evaluate the new interfaces. Maybe we could another reviewer to look it over?

The only thing that struck me as odd was the casing of the V2 and V3 files. I think it's more typical for all file names to be lower case.