require encoder for SentryOutput

As a best practice, I'm converting outputs to use encoders. Setting `PayloadEncoder` keeps the previous behavior for all Outputs I've modified this way, so far.

This lets us also use a FieldEncoder to remap fields via configuration in Heka toml: https://github.com/Clever/heka-clever-plugins/pull/9

Most immediately I want to use this to write the SentryDSN via FieldEncoder when the message gets to the `SentryOutput`, instead of needing to have the SentryDSN written during the decoding step, which we're doing now.

Yes - an encoder mutates the message, and we want to use data from fields besides just Payload (which is what's returned).

So if I want to say, set the `dsn` in the message, I need to Encode it and then refer to that message field