[COMMITTERS] pgsql: Add max_sync_workers_per_subscription topostgresql.conf.sample.

Add max_sync_workers_per_subscription to postgresql.conf.sample.

This commit also does

- add REPLICATION_SUBSCRIBERS into config_group
- mark max_logical_replication_workers and max_sync_workers_per_subscription
  as REPLICATION_SUBSCRIBERS parameters
- move those parameters into "Subscribers" section in postgresql.conf.sample

Author: Masahiko Sawada, Petr Jelinek and me
Reported-by: Masahiko Sawada
Discussion: http://postgr.es/m/CAD21AoAonSCoa=v=87ZO3vhfUZA1k_E2XRNHTt=xioWGUa+0ug@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/ff7bce174390ae063c122ab04020bd4ee070d6ad

Modified Files
--------------
src/backend/utils/misc/guc.c                  | 6 ++++--
src/backend/utils/misc/postgresql.conf.sample | 8 +++++++-
src/include/utils/guc_tables.h                | 1 +
3 files changed, 12 insertions(+), 3 deletions(-)