Add max_frame_size option for websocket handlers

Option allows to limit a frame by size before decoding its payload.

LH: I have added a test for when the limit is reached on a nofin
fragmented frame (the last commit addressed that case but it had
no test). I have fixed formatting and other, and changed the
default value to infinity since it might otherwise be incompatible
with existing code. I also added documentation and a bunch of other
minor changes.

doc/src/guide/ws_handlers.asciidoc

@@ -225,6 +225,22 @@ init(Req, State) ->
 This value cannot be changed once it is set. It defaults to
 `60000`.
 
+=== Limiting frame sizes
+
+Cowboy accepts frames of any size by default. You should
+limit the size depending on what your handler may handle.
+You can do this via the `init/2` callback:
+
+[source,erlang]
+----
+init(Req, State) ->
+    {cowboy_websocket, Req, State, #{
+        max_frame_size => 8000000}}.
+----
+
+The lack of limit is historical. A future version of
+Cowboy will have a more reasonable default.
+
 === Saving memory
 
 The Websocket connection process can be set to hibernate

doc/src/manual/cowboy_websocket.asciidoc

@@ -153,6 +153,7 @@ Cowboy does it automatically for you.
 opts() :: #{
     compress => boolean(),
     idle_timeout => timeout(),
+    max_frame_size => non_neg_integer() | infinity,
     req_filter => fun((cowboy_req:req()) -> map())
 }
 ----
@@ -181,6 +182,13 @@ idle_timeout (60000)::
     connection open without receiving anything from
     the client.
 
+max_frame_size (infinity)::
+    Maximum frame size allowed by this Websocket
+    handler. Cowboy will close the connection when
+    a client attempts to send a frame that goes over
+    this limit. For fragmented frames this applies
+    to the size of the reconstituted frame.
+
 req_filter::
     A function applied to the Req to compact it and
     only keep required information. The Req is only