Allow passing options to sub protocols

Before this commit we had an issue where configuring a
Websocket connection was simply not possible without
doing magic, adding callbacks or extra return values.
The init/2 function only allowed setting hibernate
and timeout options.

After this commit, when switching to a different
type of handler you can either return

  {module, Req, State}

or

  {module, Req, State, Opts}

where Opts is any value (as far as the sub protocol
interface is concerned) and is ultimately checked
by the custom handlers.

A large protocol like Websocket would accept only
a map there, with many different options, while a
small interface like loop handlers would allow
passing hibernate and nothing else.

For Websocket, hibernate must be set from the
websocket_init/1 callback, because init/2 executes
in a separate process.

Sub protocols now have two callbacks: one with the
Opts value, one without.

The loop handler code was largely reworked and
simplified. It does not need to manage a timeout
or read from the socket anymore, it's the job of
the protocol code. A lot of unnecessary stuff was
therefore removed.

Websocket compression must now be enabled from
the handler options instead of per listener. This
means that a project can have two separate Websocket
handlers with different options. Compression is
still disabled by default, and the idle_timeout
value was changed from inifnity to 60000 (60 seconds),
as that's safer and is also a good value for mobile
devices.

src/cowboy_http.erl

@@ -136,11 +136,10 @@ init(Parent, Ref, Socket, Transport, Opts) ->
 %% Timeouts:
 %% - waiting for new request (if no stream is currently running)
 %%   -> request_timeout: for whole request/headers, set at init/when we set ps_request_line{} state
-%% - waiting for body (if a stream requested the body to be read)
-%%   -> read_body_timeout: amount of time we wait without receiving any data when reading the body
+%% - waiting for new request, or body (when a stream is currently running)
+%%   -> idle_timeout: amount of time we wait without receiving any data
 %% - if we skip the body, skip only for a specific duration
 %%   -> skip_body_timeout: also have a skip_body_length
-%% - none if we have a stream running and it didn't request the body to be read
 %% - global
 %%   -> inactivity_timeout: max time to wait without anything happening before giving up