DoorAttendant should accept more than TcpStreams

[robertdfrench]

Is your feature request related to a problem? Please describe.
The only thing that can be sent through an application door is a TcpStream.

Describe the solution you'd like
Would like to be able to send data instead, or possibly data and a TcpStream. For example, the parsed headers could be sent as data, and the remaining TcpStream could be sent as a file descriptor so that the application door server can read it as the body. This will allow portunus to route the request based on the headers, but without copying the entire request body into its own memory first.

Describe alternatives you've considered
Alternatives are what is currently implented -- passing only a TcpStream. That may still be fine for the case where we want an unprivileged process to be able to handle streams without being able to bind to a port (are there any such cases?)

Additional context
Add any other context or screenshots about the feature request here.