xrpc should support filter-style Handlers with a way to pass context

[jkinkead]

We need to reimplement Netty pipelines have a user-friendly way to define simple filter handlers that are applied to inbound XrpcRequest objects.

[pdex]

Do we want to borrow from finagle here?

https://twitter.github.io/finagle/guide/ServicesAndFilters.html

[jkinkead]

That makes me glad we're not using Scala.

But yes, that's more or less what I was picturing.

[xjdr]

+1 for adopting finagle style service filtering