Tests for INSERT ... ON CONFLICT {UPDATE | IGNORE}

Add dedicated isolation tests for both UPDATE and IGNORE variants,
illustrating the "MVCC violation" that allows a READ COMMITTED
transaction's UPDATE to succeed in updating a tuple with no version
visible to its command's MVCC snapshot.  Add regression tests, which for
the most part are intended to exercise interactions with other features
(e.g.  updatable views, inheritance, triggers).

Add a few general purpose smoke tests too, testing everything from
EXPLAIN output to unique index inference.
Internal documentation for INSERT ... ON CONFLICT {UPDATE | IGNORE}

Includes documentation for both the nbtree and executor READMEs, as well
as the user-facing SGML documentation around index AMs.  The latter two
additions should make clear how the AM interface has been revised for
amcanunique AMs in the abstract, without reference to the only current
implementation, nbtree.
User-visible documentation for INSERT ... ON CONFLICT {UPDATE | IGNORE}

INSERT ... ON CONFLICT {UPDATE | IGNORE} is documented as a new clause
of the INSERT command.  Some potentially surprising interactions with
triggers are noted -- BEFORE INSERT per-row triggers must fire without
the INSERT path necessarily being taken, for example.

All the existing features that INSERT ... ON CONFLICT {UPDATE | IGNORE}
fails to completely play nice with have those limitations noted.  (Notes
are added to the existing documentation for those other features,
although some of these cases will need to be revisited).  This includes
postgres_fdw, updatable views and table inheritance (the latter two have
most interesting cases covered).  In principle it is the responsibility
of writable foreign data wrapper authors to provide appropriate support
for this new clause.  As such, unlike postgres_fdw itself, writable
foreign table wrappers in the abstract are supported in that a FDW
author could do something special with a ModifyTable plan that used
speculative insertion.  This theory may be put to the test before too
long.

Finally, a user-level description of the new "MVCC violation" that the
ON CONFLICT UPDATE variant sometimes requires has been added to "Chapter
13 - Concurrency Control", beside existing commentary on READ COMMITTED
mode's special handling of concurrent updates.  The new "MVCC violation"
introduced seems somewhat distinct from the existing one (i.e.  what is
internally referred to as the EvalPlanQual() mechanism), because in READ
COMMITTED mode it is no longer necessary for any row version to be
conventionally visible to the command's MVCC snapshot for an UPDATE of
the row to occur (or for the row to be locked, should the WHERE clause
predicate not be satisfied).