add Send to batch_get_or_else (#297)

Signed-off-by: Joe Zhou <[email protected]>

src/transaction/buffer.rs

@@ -69,7 +69,7 @@ impl Buffer {
         f: F,
     ) -> Result<impl Iterator<Item = KvPair>>
     where
-        F: FnOnce(Box<dyn Iterator<Item = Key>>) -> Fut,
+        F: FnOnce(Box<dyn Iterator<Item = Key> + Send>) -> Fut,
         Fut: Future<Output = Result<Vec<KvPair>>>,
     {
         let (cached_results, undetermined_keys) = {