Release notes for next week's back-branch releases

I got the first draft of $SUBJECT done a little earlier than usual.
See

https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=a2e0cf45c21afbcbc544d1aca8d51d90004aa5d9

There seemed to be more than the usual quota of commits that I decided
not to document because they seemed uninteresting to end users, such
as test-only changes.  If you think I omitted anything that should be
documented, don't hesitate to say so.

Please send any corrections before Sunday.

            regards, tom lane