Fix clientXLogPos handling in pglogrepl_demo

[espadolini]

This PR changes the handling of clientXLogPos to match the one from Postgres' own pg_recvlogical:

• keepalive messages should bump the position too, as they're only sent (from what I can tell) after any xlogdata message;
• both the WALStart and the ServerWALEnd in logical xlogdata messages represent the position that should be reported back, and adding the length of the post-decoding data to it is meaningless;
• relation messages have a position of zero, and in general we should match the pg_recvlogical behavior of only increasing the local position.

[espadolini]

As a bonus, this fixes #20 for me; without bumping the local position on keepalive, a shutting down Postgres just keeps spamming my client with keepalive messages for 10 seconds before hard disconnecting the replication connection.

[jackc]

👍