Disallow direct use of the pgrepack logical decoding plugin

Disallow direct use of the pgrepack logical decoding plugin

Nothing is to be gained from using pgrepack outside of REPACK
(CONCURRENTLY), and it leads to assertion failures in assertion-enabled
builds, and to crashes due to bogus memory lifetime in production
builds.  Reject attempts to do that with a clean error report.

Clean up the nearby code a tad while at it.  The only functional changes
in that are that the output_writer_private context is allocated and
partially filled by the pgrepack output plugin; and that ->relid therein
is now always present (rather than only in assertion-enabled builds).
Other than that it's just minor code rearrangement and added comments.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reported-by: Nikita Kalinin <n.kalinin@postgrespro.ru>
Suggested-by: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: Antonin Houska <ah@cybertec.at> (older version)
Reviewed-by: Srinath Reddy Sadipiralla <srinath2133@gmail.com> (older version)
Discussion: https://postgr.es/m/19500-38a02529a69353a5@postgresql.org