Remove BodyReader::is_deferred (API Change):

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.

test/exemplars.cpp

@@ -48,14 +48,6 @@ struct Body_BodyReader {
 struct BodyReader
 {
 public:
-    /** Controls when the implementation requests buffers.
-
-        If false, the implementation will request the first buffer
-        immediately and try to serialize both the header and some
-        or all of the body in a single buffer.
-    */
-    using is_deferred = std::false_type;
-
     /// The type of buffer returned by `get`.
     using const_buffers_type = boost::asio::const_buffers_1;