MINOR: cache: report the number of cache lookups and cache hits

The cache lookups and hits is now accounted per frontend and per backend,
and reported on the stats page.
diff --git a/include/types/stats.h b/include/types/stats.h
index c3b9fc8..ec29c78 100644
--- a/include/types/stats.h
+++ b/include/types/stats.h
@@ -394,6 +394,8 @@
 	ST_F_WREW,
 	ST_F_CONNECT,
 	ST_F_REUSE,
+	ST_F_CACHE_LOOKUPS,
+	ST_F_CACHE_HITS,
 
 	/* must always be the last one */
 	ST_F_TOTAL_FIELDS