Use new limit to control usage of bitvector query time.

Use bitvector if it exists, resulting in degraded bm25 score contribution.
Otherwise use posocc posting list with full bm25 score contribution.

tests/performance/mmap_vs_directio/app/schemas/wikimedia.sd

@@ -27,11 +27,10 @@ schema wikimedia {
     }
   }
 
-  rank-profile use_bitvectors {
-    rank title: filter
-    rank text: filter
-    first-phase {
-      expression: attribute(id)
+  rank-profile use_bitvectors inherits default {
+    rank-properties {
+      # This is set to 1/64 which is the limit for generating bitvectors.
+      vespa.matching.diskindex.bitvector_limit: 0.015625
     }
   }