Implement "fastupdate" support for GIN indexes, in which we try to accumulate

Implement "fastupdate" support for GIN indexes, in which we try to accumulate
multiple index entries in a holding area before adding them to the main index
structure.  This helps because bulk insert is (usually) significantly faster
than retail insert for GIN.

This patch also removes GIN support for amgettuple-style index scans.  The
API defined for amgettuple is difficult to support with fastupdate, and
the previously committed partial-match feature didn't really work with
it either.  We might eventually figure a way to put back amgettuple
support, but it won't happen for 8.4.

catversion bumped because of change in GIN's pg_am entry, and because
the format of GIN indexes changed on-disk (there's a metapage now,
and possibly a pending list).

Teodor Sigaev