Add CONCURRENTLY option to REPACK
 
When this flag is specified, REPACK no longer acquires access-exclusive
lock while the new copy of the table is being created; instead, it
creates the initial copy under share-update-exclusive lock only (same as
vacuum, etc), and it follows an MVCC snapshot; it sets up a replication
slot starting at that snapshot, and uses a concurrent background worker
to do logical decoding starting at the snapshot to populate a stash of
concurrent data changes.  Those changes can then be re-applied to the
new copy of the table just before swapping the relfilenodes.
Applications can continue to access the original copy of the table
normally until just before the swap, which is the only point at which
the access-exclusive lock is needed.
 
There are some loose ends in this commit:
1. concurrent repack needs its own replication slot in order to apply
   logical decoding, which are a scarce resource and easy to run out of.
2. due to the way the historic snapshot is initially set up, only one
   REPACK process can be running at any one time on the whole system.
3. there's a danger of deadlocking (and thus abort) due to the lock
   upgrade required at the final phase.
 
These issues will be addressed in upcoming commits.
 
The design and most of the code are by Antonin Houska, heavily based on
his own pg_squeeze third-party implementation.
 
Author: Antonin Houska <ah@cybertec.at>
Co-authored-by: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Co-authored-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com>
Reviewed-by: Srinath Reddy Sadipiralla <srinath2133@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Reviewed-by: Robert Treat <rob@xzilla.net>
Reviewed-by: Noriyoshi Shinoda <noriyoshi.shinoda@hpe.com>
Reviewed-by: vignesh C <vignesh21@gmail.com>
Discussion: https://postgr.es/m/5186.1706694913@antos
Discussion: https://postgr.es/m/202507262156.sb455angijk6@alvherre.pgsql