also move WAL file when moving database (#2583)

the WAL files come from sqlite WAL mode,
they have the same name as the database file with an extra `-wal` suffix.

according to https://sqlite.org/wal.html#the_wal_file ,
the WAL file is part of the persistent state of the database
and should be kept with the database if the database is copied or moved.

src/context.rs

@@ -559,6 +559,13 @@ impl Context {
         blob_fname.push("-blobs");
         dbfile.with_file_name(blob_fname)
     }
+
+    pub fn derive_walfile(dbfile: &PathBuf) -> PathBuf {
+        let mut wal_fname = OsString::new();
+        wal_fname.push(dbfile.file_name().unwrap_or_default());
+        wal_fname.push("-wal");
+        dbfile.with_file_name(wal_fname)
+    }
 }
 
 impl InnerContext {