replication protocol v4 support

[jay-babu]

I am just starting out in using this library. There is one thing I noticed that doesn't seem to be answered. The library only seems to support v2 of the replication protocol. v4 seems to be the latest. any plans on supporting the latest: https://www.postgresql.org/docs/current/protocol-logical-replication.html.

[jay-babu]

#68
i see this but no activity since last year

[jay-babu]

Happy to create a PR if this issue is accepted

[jackc]

See #83. I'd be happy to have v4 support, but I'm not able to able to adequately review that type feature now.