fix(lsm): store data len in db; fix bug

lsm/include/lsm/store.h

@@ -91,6 +91,14 @@ typedef struct lsm_store lsm_store;
  */
 lsm_error lsm_store_init(lsm_store **ptr);
 
+/**
+ * Return how many elements are stored in the trie.
+ *
+ * @param store store to use
+ * @return how many elements are in the store
+ */
+uint64_t lsm_store_size(lsm_store *store);
+
 /**
  * Open the given database file and load it into a new store object.
  *