open items list cleanup

I spent much of today going through here:

https://wiki.postgresql.org/wiki/PostgreSQL_9.5_Open_Items

Here's what I did:

* Committed patches for four of the items, hopefully resolving those items.
* Moved three items from "open" to either "resolved" or a new section
"don't need fixing".
* Added several new items based on recent posts to pgsql-hackers.
* Split up the remaining open items into sections.
* Added a comment with current status to many, but not all, of the
items. (I would have done them all, but I'm pretty much out of time
to work on this for today.)

There are a sizeable number of items on that list for which no patches
exist yet. It would be great if the people involved in those issues
could work on producing those patches. There are a few issues on that
list that require a committer's attention to a patch that has already
been produced. I believe that the onus is on the committers who
committed those features to look at those patches - or if not, they
should be up-front about the fact that they no longer have time to
work on the features they committed, so that other committers know
that they need to and should step in.

Further improvements to the wiki page itself are also welcome and will
be appreciated, at least, by me.

Thanks,

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company