unique indexes on partitioned tables

This is the patch series for UNIQUE / PRIMARY KEY indexes on partitioned
tables.  This is on top of the patch in
https://postgr.es/m/20171229175930.3aew7lzwd5w6m2x6@alvherre.pgsql
but I included it here as 0001 for simplicity.  (Don't review that patch
in this thread please).  This is essentially the same patch I posted
elsewhere in that thread.

I included Amit's support for ON CONFLICT DO UPDATE, but as I mentioned
in the other thread, it has a small bug.  In principle we could push
0002 together with 0003, but I'd rather fix 0004 first and push it all
as one commit.

This serves as basis to build foreign keys on top; I'll post that
separately.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services