routing: add support for chunked requests

[jrudolph]

Message protocols for normal requests (HttpRequest) and chunked requests (ChunkedRequestStart / MessageChunk / ChunkedMessageEnd) differ quite a bit. Therefore, spray-routing needs to be improved to handle chunked requests similar to other request.

What we need to do is:

• change all request-targeted directives in a way that allows them also to filter on
  headers coming in from a ChunkedRequestStart
• change directives regarding request entities to automatically either aggregate chunks or reject chunked requests
• add directives to handle request entities in a streaming manner

This is a sub-issue of #281.

[sirthias]

Fixed in Akka HTTP with the new streamified chunking model.