Support INSERT ... ON CONFLICT IGNORE

Support INSERT ... ON CONFLICT 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 IGNORE path may be taken
(the tuple slot proposed for insertion is skipped without raising an
error).  The implementation loops until either an insert occurs, or a
conclusively committed conflicting tuple is determined to exist.

This is implemented using a new infrastructure called "speculative
insertion".  (The approach to "Value locking" presenting here follows
design #2, as described on the value locking Postgres Wiki page).

Users optionally specify a single unique index to take the alternative
path on, which is inferred from a set of user-supplied column names (or
expressions).

Speculative (heap) insertions are WAL-logged in two steps:  One record
relates to an initial intent to insert, while a second minimal record
simply affirms that that attempt was ultimately successful (i.e. no
conflicts where detected when inserting into constraint-related
indexes).  In this revision, logical decoding does not rely on the
presence of this second record to affirm that a speculative insertion
succeeded, though; it relies on the *absence* on an (internal) "super
deletion" record.