HTTP: Add Cluster support

Via @nickdaugherty in https://github.com/Automattic/vip-go-node/pull/8#issuecomment-410760943

> Oh another consideration, and something we don't necessarily have to support in this PR, is using Node Cluster for more resilient and performant apps out of the box.
>
> The library could abstract away and hide the details of setting up a clustered process, though maybe should only be enabled with an option since it requires the application to be written with multi process in mind.
>
> For an example of how we're using Cluster on Go now (but sadly only using a single worker), see the public API repo.

Start by reading the library's HTTP entry points and the public API repository's Node Cluster example referenced in the issue. Clarify the optional configuration, worker behavior, and application assumptions before implementation. Done should include documented Cluster support and coverage for clustered and default single-process operation.

Written by the indexing model from the issue text.