Unique indexes & constraints on partitioned tables

Hello,

I'm giving this patch its own thread for mental sanity, but this is
essentially what already posted in [1], plus some doc fixes. This patch
depends on the main "local partitioned indexes" in that thread, last
version of which is at [2].

I also added a mechanism to set the constraints in partitions as
dependent on the constraint in the parent partitioned table, so deletion
is sensible: the PK in partitions goes away when the PK in the parent
table is dropped; and you can't drop the PK in partitions on their own.

In order to implement that I dress Rube Goldberg as Santa: the
constraint OID of the parent is needed by index_constraint_create when
creating the child; but it's the previous index_constraint_create itself
that generates the OID when creating the parent, and it's DefineIndex
that does the recursion. So index_constraint_create returns the value
to index_create who returns it to DefineIndex, so that the recursive
step can pass it down to index_create to give it to
index_constraint_create. It seems crazy, but it's correct.

As far as I can tell, pg_dump works correctly without any additional
changes.

[1] https://postgr.es/m/20171220194937.pldcecyx7yrwmgkg@alvherre.pgsql
[2] https://postgr.es/m/20171220212503.aamhlrs425flg47f@alvherre.pgsql

--
Álvaro Herrera https://www.2ndQuadrant.com/
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