The is_deferred nested type is removed from the BodyReader
requirements.
Performance for sending messages with `file_body` was cut
almost in half with is_deferred as `std::true_type` since
it caused double the number of socket writes. If the
deferred behavior is absolutely necessary, callers can get
the same effect by manually sending the headers first.
Actions Required:
* Callers who need the behavior of is_deferred as `std::true_type`
should manually construct a serializer and serialize the header
first, followed by the body.
* FieldsReader now requires chunked() and keep_alive()
* serializer logic calls into the reader to calculate
the message metadata
* Removed some of the previous requirements of FieldsReader
Actions Required:
* Implement chunked() and keep_alive() for user defined FieldsReader types.
Vinnie Falco