Re: allow online change primary_conninfo

Hello

> Yeah, you are right. I was not paying much attention but something
> does not stick here. My understanding is that we should have the WAL
> receiver receive the value it needs to use from the startup process
> (aka via RequestXLogStreaming from xlog.c), and that we ought to make
> this new parameter PGC_POSTMASTER instead of PGC_SIGHUP. HEAD is
> inconsistent here.

Thank you!

I attached two patches:
- first changes wal_receiver_create_temp_slot to PGC_POSTMASTER and moved the logic to RequestXLogStreaming
- second is based on last published v6 version of main patch. It changes wal_receiver_create_temp_slot back to PGC_SIGHUP along with primary_conninfo and primary_slot_name and will restart walreceiver if need.

Regarding the main patch: we have several possibilities for moving RequestXLogStreaming call. We need to choose one.
And, of course, changes in the text...

regards, Sergei