First-draft release notes for next week's back-branch releases

I've done a first pass at next week's release notes; please review.

Committed at
http://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=7008e70d105b572821406744ce080771b74c06ab
and should be visible in the website's devel-branch docs after the next
guaibasaurus buildfarm run, due a couple hours from now.

As usual, I just dumped all the notes into one branch's release-N.N.sgml
file, and will sort out which ones apply to which branches later.  I chose
to put them into 9.4, though, not 9.5, since many of these issues are
already fixed in 9.5.0 and will not need to appear in the 9.5.1 section.
        regards, tom lane