add migration code

src/context.rs

@@ -458,6 +458,13 @@ impl Context {
         self.get_config(Config::ConfiguredMvboxFolder).await
             == Some(folder_name.as_ref().to_string())
     }
+
+    pub fn derive_blobdir(dbfile: &PathBuf) -> PathBuf {
+        let mut blob_fname = OsString::new();
+        blob_fname.push(dbfile.file_name().unwrap_or_default());
+        blob_fname.push("-blobs");
+        dbfile.with_file_name(blob_fname)
+    }
 }
 
 impl InnerContext {