Synchronizing slots from primary to standby

I want to reactivate $subject.  I took Petr Jelinek's patch from [0], 
rebased it, added a bit of testing.  It basically works, but as 
mentioned in [0], there are various issues to work out.

The idea is that the standby runs a background worker to periodically 
fetch replication slot information from the primary.  On failover, a 
logical subscriber would then ideally find up-to-date replication slots 
on the new publisher and can just continue normally.

The previous thread didn't have a lot of discussion, but I have gathered 
from off-line conversations that there is a wider agreement on this 
approach.  So the next steps would be to make it more robust and 
configurable and documented.  As I said, I added a small test case to 
show that it works at all, but I think a lot more tests should be added. 
  I have also found that this breaks some seemingly unrelated tests in 
the recovery test suite.  I have disabled these here.  I'm not sure if 
the patch actually breaks anything or if these are just differences in 
timing or implementation dependencies.  This patch adds a LIST_SLOTS 
replication command, but I think this could be replaced with just a 
SELECT FROM pg_replication_slots query now.  (This patch is originally 
older than when you could run SELECT queries over the replication protocol.)

So, again, this isn't anywhere near ready, but there is already a lot 
here to gather feedback about how it works, how it should work, how to 
configure it, and how it fits into an overall replication and HA 
architecture.


[0]: 
https://www.postgresql.org/message-id/flat/3095349b-44d4-bf11-1b33-7eefb585d578%402ndquadrant.com