GELF Input falsely complains about invalid timestamp field

Receiving a GELF message with a timestamp as string will trigger a validation warrning:

https://github.com/Graylog2/graylog2-server/blob/master/graylog2-server/src/main/java/org/graylog2/inputs/codecs/GelfCodec.java#L273

However, these messages can be processed just fine:
https://github.com/Graylog2/graylog2-server/blob/master/graylog2-server/src/main/java/org/graylog2/inputs/codecs/GelfCodec.java#L111

I suggest we unify the validation and parsing methods.

While there, we might also take a look at https://github.com/Graylog2/graylog2-server/issues/8416

[#z 4471]