Re: Synchronizing slots from primary to standby

PFA v20. The changes are:

1) The launcher now checks hot_standby_feedback (along with presence
of physical slot) before launching slot-sync workers and skips the
sync if it is off.

2) Other validity checks (for primary_slot_name, dbname in
primary_conn_info etc) are now moved to the launcher before we even
launch slot-sync workers. This will fix frequent WARNING msg coming in
log file as reported by Bertrand.

3) Now we stop all the slot-sync workers in case any of the related
GUCs has changed and then relaunch these in next sync-cycle as per new
values and after performing validity checks again.

4) This patch also fixes few bugs in wait_for_primary_slot_catchup():
4.1) This function was not coming out of wait gracefully on standby's
promotion, it is fixed now.
4.2) The checks to start the wait were not correct. These have been fixed now
4.3) If the slot (on which we are waiting) is invalidated on primary
meanwhile, this function was not handling that scenario and was not
aborting the wait. Handled now.

5) Addressed most of the comments(dated Sep25) given by Kruoda-san in
patch 0001.

First 4 changes are in patch002 while last one is in patch001.

thanks
Shveta