json: output messages in JSONv1 format

json: output messages in JSONv1 format

While Logstash is quite flexible in the JSON messages received, the
canonical format it "expects" is the JSON Event v1 format. The timestamp
should be keyed by `@timestamp` and there should be a `@version` key
whose value is 1. All other keys are free.

There is no formal specification of this format. It is however described
here:

 https://github.com/elastic/logstash/blob/1.5/lib/logstash/event.rb#L26-L47

It's useful to respect this format as it allows a user to use a less
capable receiver. The new format is enabled only when `eventv1=1` is set
in plugin configuration.

Signed-off-by: Vincent Bernat <Vincent.Bernat@exoscale.ch>