Re: Synchronizing slots from primary to standby

PFA v22 patch-set. It has below changes:

patch 001:
1) Now physical walsender wakes up logical walsender(s) by using a new
CV as suggested in [1]
2) Now pg_logical_slot_get_changes (and other such get/peek functions)
as well wait for standby(s) confirmation.

patch 002:
1) New column (synced_slot) added in pg_replication_slots to indicate
if it is a synced slot or user one.
2) Any attempt to do pg_drop_replication_slot() on synced-slot will
result in an error
3) Some portion of Peter's comments dated Oct4 and Kuroda-san's
comments dated Oct 2.

Thanks Hou-san for working on changes of patch 001.

[1]: https://www.postgresql.org/message-id/a539e247-30c8-4d5c-b561-07d0949cc960%40gmail.com

thanks
Shveta