pgsql: Rename standby_slot_names to synchronized_standby_slots.

Rename standby_slot_names to synchronized_standby_slots.

The standby_slot_names GUC allows the specification of physical standby
slots that must be synchronized before the logical walsenders associated
with logical failover slots. However, for this purpose, the GUC name is
too generic.

Author: Hou Zhijie
Reviewed-by: Bertrand Drouvot, Masahiko Sawada
Backpatch-through: 17
Discussion: https://postgr.es/m/ZnWeUgdHong93fQN@momjian.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2357c9223b710c91b0f05cbc56bd435baeac961f

Modified Files
--------------
doc/src/sgml/config.sgml | 12 +--
doc/src/sgml/func.sgml | 4 +-
doc/src/sgml/logical-replication.sgml | 2 +-
doc/src/sgml/logicaldecoding.sgml | 8 +-
src/backend/replication/logical/slotsync.c | 4 +-
src/backend/replication/slot.c | 119 +++++++++++----------
src/backend/replication/walsender.c | 4 +-
src/backend/utils/misc/guc_tables.c | 6 +-
src/backend/utils/misc/postgresql.conf.sample | 4 +-
src/include/replication/slot.h | 4 +-
src/include/replication/walsender_private.h | 4 +-
src/include/utils/guc_hooks.h | 6 +-
.../recovery/t/040_standby_failover_slots_sync.pl | 48 +++++----
src/tools/pgindent/typedefs.list | 2 +-
14 files changed, 115 insertions(+), 112 deletions(-)