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.

(it's also possible that an `Encoder` might produce something that is a combination of multiple fields, e.g. `"hello, %field1% %field2%"` but I don't think they're supposed to modify the message)