more similar signatures for persistent and ephemeral command handlers
more similar signatures for persistent and ephemeral command handlers

Summary:
nonparallelizable handlers mutate `env` so they have to return it, along with the response and extra logging JSON: `(env, response, json)`

parallelizable handlers can't mutate `env` so instead they returned `((), response, json)`. the extra `()` is unnecessary.

ephemeral commands already returned just `(response, json)` so this makes persistent handlers more similar.

Changelog: [internal]

Reviewed By: samwgoldman

Differential Revision: D32294379

fbshipit-source-id: 1a5f7ea28cff8888e119001526b611396e9fbaa8

mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow