Add support for loops in standard HTTP handlers

Now init/3 can return one of the following values to enable loops:
 - {loop, Req, State}
 - {loop, Req, State, hibernate}
 - {loop, Req, State, Timeout}
 - {loop, Req, State, Timeout, hibernate}

Returning one of these tuples will activate looping in the HTTP handler.
When looping, handle/2 is never called. Instead, Cowboy will listen
for Erlang messages and forward them to the info/3 function of the
handler. If a timeout is defined, Cowboy will also close the connection
when no message has been received for Timeout milliseconds.

The info/3 function is defined as info(Msg, Req, State). It can return
either of the following tuples:
 - {ok, Req, State}
 - {loop, Req, State}
 - {loop, Req, State, hibernate}

The first one ends the connection, calling terminate/2 before closing.
The others continue the loop.

Loops are useful when writing long-polling handlers that need to wait
and don't expect to receive anything. Therefore it is recommended to
set a timeout to close the connection if nothing arrives after a while
and to enable hibernate everywhere.

Normal HTTP handlers shouldn't need to use this and as such info/3
was made optional.

test/http_handler_long_polling.erl

@@ -0,0 +1,22 @@
+%% Feel free to use, reuse and abuse the code in this file.
+
+-module(http_handler_long_polling).
+-behaviour(cowboy_http_handler).
+-export([init/3, handle/2, info/3, terminate/2]).
+
+init({_Transport, http}, Req, _Opts) ->
+	erlang:send_after(500, self(), timeout),
+	{loop, Req, 9, 5000, hibernate}.
+
+handle(_Req, _State) ->
+	exit(badarg).
+
+info(timeout, Req, 0) ->
+	{ok, Req2} = cowboy_http_req:reply(102, [], [], Req),
+	{ok, Req2, 0};
+info(timeout, Req, State) ->
+	erlang:send_after(500, self(), timeout),
+	{loop, Req, State - 1, hibernate}.
+
+terminate(_Req, _State) ->
+	ok.