Add protocol property to the transport

Transport should be protocol aware because it should be possible to
switch the protocol if the operation is supported, for example for upgrading
HTTP to web sockets or HTTP 1.1 to HTTP/2.

source/tanya/async/transport.d

@@ -3,13 +3,14 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 /**
- * Copyright: Eugene Wissner 2016.
+ * Copyright: Eugene Wissner 2016-2017.
  * License: $(LINK2 https://www.mozilla.org/en-US/MPL/2.0/,
  *                  Mozilla Public License, v. 2.0).
  * Authors: $(LINK2 mailto:info@caraus.de, Eugene Wissner)
  */
 module tanya.async.transport;
 
+import tanya.async.protocol;
 import tanya.network.socket;
 
 /**
@@ -45,6 +46,33 @@ interface WriteTransport : Transport
  */
 interface DuplexTransport : ReadTransport, WriteTransport
 {
+	/**
+	 * Returns: Application protocol.
+	 *
+	 * Postcondition: $(D_INLINECODE protocol !is null)
+	 */
+	@property Protocol protocol() pure nothrow @safe @nogc
+	out (protocol)
+	{
+		assert(protocol !is null);
+	}
+
+	/**
+	 * Switches the protocol.
+	 *
+	 * The protocol is deallocated by the event loop, it should currently be
+	 * allocated with $(D_PSYMBOL MmapPool).
+	 *
+	 * Params:
+	 * 	protocol = Application protocol.
+	 *
+	 * Precondition: $(D_INLINECODE protocol !is null)
+	 */
+	@property void protocol(Protocol protocol) pure nothrow @safe @nogc
+	in
+	{
+		assert(protocol !is null);
+	}
 }
 
 /**
@@ -52,7 +80,7 @@ interface DuplexTransport : ReadTransport, WriteTransport
  */
 interface SocketTransport : Transport
 {
-	@property inout(Socket) socket() inout pure nothrow @safe @nogc;
+	@property Socket socket() pure nothrow @safe @nogc;
 }
 
 /**