Clean Flash cookies in Akka HTTP server
Convert Result body into Akka HTTP Empty, Strict or Chunked entities

Previously all results were always Chunked.

- Refactored the buffering code out of NettyResultStreamer and moved
  it into ServerResultUtils so that the Akka HTTP server code
  could use it.
- Simplified the buffering code and added two new methods to
  Iteratee.
- Added tests for ServerResultUtils.cleanFlashCookie.
Merge pull request #3732 from richdougherty/akka-http-integration

Fixes to Akka HTTP server