[check_postgres] some fixes and a check for replication slots

[Glyn Astill]

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:

    https://github.com/bucardo/check_postgres/pull/108

I also added a check to monitor the size of WAL retained for specific replication slots:

    https://github.com/bucardo/check_postgres/pull/109

And a fix to stop replicate_row returing "values are not the same" when the repinfo values are zeros:

    https://github.com/bucardo/check_postgres/pull/110

Currently I'm having to apply these on top of the official check_postgres, so if anyone has any time it'd be great to get some pointers on what I can do to get these merged in.

Thanks
Glyn