HTTP: Add Cluster support

[mjangda]

Via @nickdaugherty in #8 (comment)

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.

[mjangda]

Continued in #41

[nickdaugherty]

Oops, I guess I missed this one when opening #41

[nickdaugherty]

Oh actually, #41 was for Logging specifically (really just logging which worker an event occurred on). This issue was for adding Cluster to the core server itself.

[mjangda]

Ah, got it.

[nickdaugherty]

When implementing this, we need to consider websocket support, which requires sticky routing to the same worker for the "polling" transport (default mode). That needs to be implemented in a performant and reliable way (public API depends on it if we want to use this base server for that...and we do).