Rethink original decision to use AND/OR Expr nodes to represent bitmap

logic operations during planning.  Seems cleaner to create two new Path
node types, instead --- this avoids duplication of cost-estimation code.
Also, create an enable_bitmapscan GUC parameter to control use of bitmap
plans.

src/backend/utils/misc/postgresql.conf.sample

@@ -137,6 +137,7 @@
 
 # - Planner Method Configuration -
 
+#enable_bitmapscan = true
 #enable_hashagg = true
 #enable_hashjoin = true
 #enable_indexscan = true