Optimize ScanIterator to avoid unnecessary copying

Previously each key/value read during iteration was getting duplicated 😮

(This doesn't strictly belong to the PR its included in, but it will
greatly benefit the DB migration script.)

Author: shesek

src/new_index/db.rs

@@ -38,8 +38,8 @@ impl<'a> Iterator for ScanIterator<'a> {
             return None;
         }
         Some(DBRow {
-            key: key.to_vec(),
-            value: value.to_vec(),
+            key: key.into_vec(),
+            value: value.into_vec(),
         })
     }
 }