Support INSERT ... ON CONFLICT {UPDATE | IGNORE}

This non-standard INSERT clause allows DML statement authors to specify
that in the event of each of any of the tuples being inserted
duplicating an existing tuple in terms of a value or set of values
constrained by a unique index, an alternative path may be taken.  The
statement may alternatively IGNORE the tuple being inserted without
raising an error, or go to UPDATE the existing tuple whose value is
duplicated by a value within one single tuple proposed for insertion.
The implementation loops until either an insert or an UPDATE/IGNORE
occurs.  No existing tuple may be affected more than once per INSERT.

This is implemented using a new infrastructure called "speculative
insertion".  A speculative inserter establishes the right to insert
ahead of time.  In order to proceed with insertion, consensus must be
reached between all unique indexes on the table, or else the alternative
path is taken.  Finally, heap tuple insertion occurs in the conventional
manner, followed by insertion of index tuples, where index tuple
insertion picks up where value locking left off, performing essentially
the conventional insertion process in a staggered fashion.
Alternatively, we may go to UPDATE, using the EvalPlanQual() mechanism
to execute a special auxiliary plan.

READ COMMITTED isolation level is permitted to UPDATE a tuple even where
no version is visible to the command's MVCC snapshot.  Similarly, any
query predicate associated with the UPDATE portion of the new statement
need only satisfy an already locked, conclusively committed and visible
conflict tuple.  When the predicate isn't satisfied, the tuple is still
locked, which implies that at READ COMMITTED, a tuple may be locked
without any version being visible to the command's MVCC snapshot.

Users may optionally specify a single unique index to merge on with a
WITHIN `unique_index` specification, which is useful when there is a
concern about spuriously merging on the wrong unique index due to there
being more than one would-be unique violation.  Otherwise, we UPDATE (or
IGNORE) based on the first would-be unique violation detected, on the
assumption that that is the only unique index where a violation could
appear.

The auxiliary ModifyTable plan used by the UPDATE portion of the new
statement is not formally a subplan of its parent INSERT ModifyTable
plan.  Rather, it's an independently planned subquery, whose execution
is tightly driven by its parent.  Special auxiliary state pertaining to
the auxiliary UPDATE is tracked by its parent through all stages of
query execution.

The optimizer imposes some restrictions on child auxiliary UPDATE plans,
which make the plans comport with their parent to the extent required
during the executor stage.  One user-visible consequences of this is
that the special auxiliary UPDATE query cannot have subselects within
its targetlist or WHERE clause.  UPDATEs may not reference any other
table, and UPDATE FROM is disallowed.  INSERT's RETURNING clause
continues to only project tuples actually inserted.
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.  Regression tests are for the
most part intended to exercise interactions with inter-related features.
Add a few general purpose smoke tests too.
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.