Phase 2 of project to make index operator lossiness be determined at runtime

instead of plan time.  Extend the amgettuple API so that the index AM returns
a boolean indicating whether the indexquals need to be rechecked, and make
that rechecking happen in nodeIndexscan.c (currently the only place where
it's expected to be needed; other callers of index_getnext are just erroring
out for now).  For the moment, GIN and GIST have stub logic that just always
sets the recheck flag to TRUE --- I'm hoping to get Teodor to handle pushing
that control down to the opclass consistent() functions.  The planner no
longer pays any attention to amopreqcheck, and that catalog column will go
away in due course.

src/backend/access/hash/hash.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/hash/hash.c,v 1.101 2008/04/10 22:25:25 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/hash/hash.c,v 1.102 2008/04/13 19:18:14 tgl Exp $
  *
  * NOTES
  *	  This file contains only the public interface routines.
@@ -210,6 +210,9 @@ hashgettuple(PG_FUNCTION_ARGS)
 	OffsetNumber offnum;
 	bool		res;
 
+	/* Hash indexes are never lossy (at the moment anyway) */
+	scan->xs_recheck = false;
+
 	/*
 	 * We hold pin but not lock on current buffer while outside the hash AM.
 	 * Reacquire the read lock here.