Get read lock when retrieving general messages

Now that the Database::general_messages() function takes &self instead of &mut self, the wrappers don't need to take a write lock to call it.

python/src/database.rs

@@ -120,7 +120,7 @@ impl Database {
         evaluate_conditions: bool,
     ) -> Result<Vec<Message>, VerboseError> {
         self.0
-            .write()
+            .read()
             .map_err(DatabaseLockPoisonError::from)?
             .general_messages(to_eval_mode(evaluate_conditions))
             .map(|v| v.into_iter().map(Into::into).collect())