Remove Async Formatters

[retrohacker]

Feature Request

Use Case

Currently the 5.x branch supports async formatters, however internally there is no way to support them and maintain the current restify API. After exploring possible solutions (i.e. #885), we have decided to drop support for async formatters prior to shipping 5x. The implications of this are that users will need to handle all asynchronous rendering outside of the restify stack, providing res.send et. al. values that are capable of being serialized synchronously.

Example API

Identical to restify 4.x formatters

Are you willing and able to implement this?

Yes 😄

[retrohacker]

Merged 🎉