Options to forward fluent tag and timestamp across amqp

I'm using AMQP to forward logs between local fluentd nodes and the central one. I want to preserve the original tag, to keep my tag schema consistent across nodes, and the timestamp, to keep log records accurate.

I added some options to assist in that.

out_amqp:
* `tag_key` will use the fluent tag as the amqp routing key
* `tag_header` will set the fluent tag as the specified amqp user header
* `time_header` will set the timestamp as the specified amqp user header, formatted as YYYY-MM-DD HH:MM:SS UTC.

in_amqp will use the same options to try to read the corresponding values. Obviously, the values must match for that to work.

All parameters are optional and will default to the previous plugin behaviour.