Pass _id from RawMessage to Message

When creating a Message from a RawMessage we throw away the already existing UUID (`_id`) and create a new one.

This is unneccessary and makes it impossible to track down messages that are logging an error during the early input processing.

See for example:
https://github.com/Graylog2/graylog2-server/blob/master/graylog2-server/src/main/java/org/graylog2/inputs/codecs/GelfCodec.java#L152

https://github.com/Graylog2/graylog2-server/blob/master/graylog2-server/src/main/java/org/graylog2/plugin/journal/RawMessage.java#L68

https://github.com/Graylog2/graylog2-server/blob/master/graylog2-server/src/main/java/org/graylog2/plugin/Message.java#L315