chore: add close db connection feature

src/database.rs

@@ -25,4 +25,7 @@ pub trait Database {
 
     /// Puts the leaves batch to the db
     fn put_batch(&mut self, subtree: HashMap<DBKey, Value>) -> PmtreeResult<()>;
+
+    /// Closes the db connection
+    fn close(&mut self) -> PmtreeResult<()>;
 }

src/tree.rs

@@ -115,6 +115,11 @@ where
         })
     }
 
+    /// Closes the db connection
+    pub fn close(&mut self) -> PmtreeResult<()> {
+        self.db.close()
+    }
+
     /// Sets a leaf at the specified tree index
     pub fn set(&mut self, key: usize, leaf: H::Fr) -> PmtreeResult<()> {
         if key >= self.capacity() {