public interface OutboundRequestHandle
ConnectionEndpoint.  The same handle is associated with all
 OutboundRequest instances produced by the same OutboundRequestIterator (that is, all attempts to send the same
 request).
 An OutboundRequestHandle is an opaque cookie
 provided to a ConnectionManager by a connection-based
 Endpoint implementation (via ConnectionManager.newRequest) in
 order to identify the request in later invocations of certain
 ConnectionEndpoint and Connection methods.  The
 handle encapsulates information about the constraints for the
 request and any other request-specific information needed by the
 transport implementation.