Instead of waiting for a later stage and "silently" dropping (logged on DEBUG)
invalid messages, `GelfCodec` now actively checks for the existence and validity
of mandatory GELF message fields (such as "version", "host", "short_message", and
"timestamp", according to the GELF spec).

Refs http://docs.graylog.org/en/2.2/pages/gelf.html
Fixes #3970

There are still many GELF client libraries out there using either "1.0" or
no value at all for the GELF "version" field.