Feature Request: make payload limit configureable

[udo1toni]

As said in https://github.com/thomasnordquist/MQTT-Explorer/issues/408, it would be nice to have this configurable.

Or (to circumvent memory limitations)

• last message is always complete
• limit historic messages to (e.g.) 20.000 Bytes each and truncate.
• at least mark truncated payloads as truncated, so a user is aware of the situation.