Re: remove wal_level archive

- The distinction between wal_level settings "archive" and "hot_standby"
is in the way of automation or better intelligence, because the primary
cannot tell what the receiver intends to do with the WAL.

So here is a patch to get rid of the distinction.

Bike-shedding:  In this patch, I removed "archive" and kept
"hot_standby", because that's what the previous discussions suggested.
Historically and semantically, it would be more correct the other way
around.  On the other hand, keeping "hot_standby" would probably require
fewer configuration files to be changed.  Or we could keep both, but
that would be confusing (for users and in the code).