Configurable payload limit for private messages

When sending private pinned messages FireFly will batch them and then send a single API call to the data exchange plugin. The issue with this approach is that the payload can get arbitrarily large. The batching mechanism should continue to accept new messages into the current batch as long as the combined payload does not exceed a configurable limit. Note also that if a single message with a payload that exceeds that limit is submitted to FireFly, it should be rejected.