tableam: New callback relation_fetch_toast_slice.

Instead of always calling heap_fetch_toast_slice during detoasting,
invoke a table AM callback which, when the toast table is a heap
table, will be heap_fetch_toast_slice.

This makes it possible for a table AM other than heap to be used
as a TOAST table. It also completes the series of commits intended
to improve the interaction of tableam with TOAST that began with
commit 8b94dab06617ef80a0901ab103ebd8754427ef5a; detoast.c is
now, hopefully, fully AM-independent.

Patch by me, reviewed by Andres Freund and Peter Eisentraut.

Discussion: http://postgr.es/m/CA+TgmoZv-=2iWM4jcw5ZhJeL18HF96+W1yJeYrnGMYdkFFnEpQ@mail.gmail.com

src/backend/access/heap/heapam_handler.c

@@ -2545,6 +2545,7 @@ static const TableAmRoutine heapam_methods = {
 	.relation_size = table_block_relation_size,
 	.relation_needs_toast_table = heapam_relation_needs_toast_table,
 	.relation_toast_am = heapam_relation_toast_am,
+	.relation_fetch_toast_slice = heap_fetch_toast_slice,
 
 	.relation_estimate_size = heapam_estimate_rel_size,