XMin Hot Standby Feedback patch

This is another bit of the syncrep patch split out.

I will revisit the replication timeout one Real Soon, I promise -- but
I have a couple things to do today that may delay that until the
evening.

https://github.com/fdr/postgres/commit/ad3ce9ac62f0e128d7d1fd20d47184f867056af1

Context diff supplied here.

Note that this information is not exposed via catalog in the original
syncrep patch, and is not here. Do we want that kind of reporting?

-- 
fdr