Rewrite DROP's dependency traversal algorithm into an honest two-pass

algorithm, replacing the original intention of a one-pass search, which
had been hacked up over time to be partially two-pass in hopes of handling
various corner cases better.  It still wasn't quite there, especially as
regards emitting unwanted NOTICE messages.  More importantly, this approach
lets us fix a number of open bugs concerning concurrent DROP scenarios,
because we can take locks during the first pass and avoid traversing to
dependent objects that were just deleted by someone else.

There is more that can be done here, but I'll go ahead and commit the
base patch before working on the options.

src/include/access/genam.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/genam.h,v 1.72 2008/05/12 00:00:53 alvherre Exp $
+ * $PostgreSQL: pgsql/src/include/access/genam.h,v 1.73 2008/06/08 22:41:04 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -138,6 +138,7 @@ extern SysScanDesc systable_beginscan(Relation heapRelation,
 				   Snapshot snapshot,
 				   int nkeys, ScanKey key);
 extern HeapTuple systable_getnext(SysScanDesc sysscan);
+extern bool systable_recheck_tuple(SysScanDesc sysscan, HeapTuple tup);
 extern void systable_endscan(SysScanDesc sysscan);
 extern SysScanDesc systable_beginscan_ordered(Relation heapRelation,
 											  Relation indexRelation,