add flush on drop to handle soft shutdown write

src/database/abstraction.rs

@@ -338,6 +338,15 @@ impl WriteCache {
     }
 }
 
+#[cfg(feature = "persy")]
+impl Drop for WriteCache {
+    fn drop(&mut self) {
+        if self.changes_count > 0 {
+            self.flush_changes().unwrap();
+        }
+    }
+}
+
 #[cfg(feature = "persy")]
 struct UnionIter<T: Iterator<Item = I>, T1: Iterator<Item = I>, I> {
     first: Peekable<T>,