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