ALTER TABLE uses a bistate but not for toast tables

ATRewriteTable() calls table_tuple_insert() with a bistate, to avoid clobbering
and polluting the buffers.

But heap_insert() then calls 
heap_prepare_insert() >
heap_toast_insert_or_update >
toast_tuple_externalize >
toast_save_datum >
heap_insert(toastrel, toasttup, mycid, options, NULL /* without bistate:( */);

I came up with this patch.  I'm not sure but maybe it should be implemented at
the tableam layer and not inside heap.  Maybe the BulkInsertState should have a
2nd strategy buffer for toast tables.

CREATE TABLE t(i int, a text, b text, c text,d text,e text,f text,g text);
INSERT INTO t SELECT 0, array_agg(a),array_agg(a),array_agg(a),array_agg(a),array_agg(a),array_agg(a) FROM generate_series(1,999)n,repeat(n::text,99)a,generate_series(1,99)b GROUP BY b;
INSERT INTO t SELECT * FROM t;
INSERT INTO t SELECT * FROM t;
INSERT INTO t SELECT * FROM t;
INSERT INTO t SELECT * FROM t;

ALTER TABLE t ALTER i TYPE smallint;
SELECT COUNT(1), relname, COUNT(1) FILTER(WHERE isdirty) FROM pg_buffercache b JOIN pg_class c ON c.oid=b.relfilenode GROUP BY 2 ORDER BY 1 DESC LIMIT 9;

Without this patch:
postgres=# SELECT COUNT(1), relname, COUNT(1) FILTER(WHERE isdirty) FROM pg_buffercache b JOIN pg_class c ON c.oid=b.relfilenode GROUP BY 2 ORDER BY 1 DESC LIMIT 9;
 10283 | pg_toast_55759                  |  8967

With this patch:
  1418 | pg_toast_16597                  |  1418

-- 
Justin