Fix truncation with async handlers

Truncation strategies were prepared to use symbolized payloads as we generate them. But we didn't take account that when the payload is sent by an async handler the payload keys are probably strings.

Ex: The payload with symbolized keys is serialized into a Redis queue, the payload is deserialized from the queue, the keys are now strings, truncation fails.

I've change the code to ensure that all the payloads have only string keys when it is going to be truncated and sent. The truncation strategies use now strings to access the data instead of symbols. This behaviour is now the same for inline reports or async reports.