[check_postgres] some fixes and a check for replication slots

[Christoph Berg]

Re: Glyn Astill 2016-06-01 <1932296388.5236885.1464796787427.JavaMail.yahoo at mail.yahoo.com>
> Hi,
> 
> A while back I had a few issues using the same_schema checks across logically replicated nodes.  These were mainly due to the checks comparing oid fields and I created a pull request with some fixes, but it gradually became a mess as I made more commits to the fork; sorry about that.  I've separated out that collection of commits here:

Hi Glyn,

to follow up here as well: thanks for the patches, and thanks for the
patience! I should have tackled these already last year.

Christoph