pageinspect: Use new pg_lsn datatype.

Michael Paquier, with slight comment changes by me
Corrections to replication slots code and documentation.

Andres Freund, per a report from Vik Faering
Bump catversion.

The previous patch should have entailed a catversion bump, but I
forgot.
Use a longer buffer in libpqrcv_startstreaming.

Because of the new SLOT clause in the START_REPLICATION command, it's
possible for the command to end up too long for the old maximum buffer
length.

Andres Freund