Please add an option to skip invalid metadata instead of shutdown the consumer

[zhengcan]

Some producer may write metadata with invalid UTF-8 characters.
It will lead the fn Codec::decode() return Err, and the consumer will shutdown and reboot immediately.
Because the invalid message is not acked, so it will cause the whole app unusable.

Currently, I have to hack the src/consumer/message.rx to ignore the invalid metadata to allow my app working as normal.
Please consider to add an option to skip the invalid metadata.

BTW: the producer is out of my control, it may or may not changed.