pgsql: Reserve replication slots specifically for REPACK

Reserve replication slots specifically for REPACK

Add a new GUC max_repack_replication_slots, which lets the user reserve
some additional replication slots for concurrent repack (and only
concurrent repack). With this, the user doesn't have to worry about
changing the max_replication_slots in order to cater for use of
concurrent repack.

(We still use the same pool of bgworkers though, but that's less
commonly a problem than slots.)

Author: Álvaro Herrera <alvherre(at)kurilemu(dot)de>
Reviewed-by: Srinath Reddy Sadipiralla <srinath2133(at)gmail(dot)com>
Discussion: https://postgr.es/m/202604012148.nnnmyxxrr6nh@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e76d8c749c3152657711ed733f0aea61c0e36a91

Modified Files
--------------
doc/src/sgml/config.sgml | 15 +++++
doc/src/sgml/ref/repack.sgml | 6 +-
src/backend/commands/repack_worker.c | 7 +-
src/backend/replication/logical/launcher.c | 2 +-
src/backend/replication/logical/logical.c | 8 ++-
src/backend/replication/logical/logicalfuncs.c | 2 +-
src/backend/replication/logical/slotsync.c | 5 +-
src/backend/replication/slot.c | 92 +++++++++++++++++---------
src/backend/replication/slotfuncs.c | 19 +++---
src/backend/replication/walsender.c | 9 +--
src/backend/utils/misc/guc_parameters.dat | 8 +++
src/backend/utils/misc/postgresql.conf.sample | 2 +
src/include/replication/logical.h | 3 +-
src/include/replication/slot.h | 5 +-
14 files changed, 121 insertions(+), 62 deletions(-)