Enforce Max Sessions

[andrewmd5]

  When an HTTP/3 connection is established, both the client and server
   have to send a SETTINGS_WEBTRANSPORT_MAX_SESSIONS setting in order to
   indicate that they both support WebTransport over HTTP/3.  This
   process also negotiates the use of additional HTTP/3 extensions.

   WebTransport sessions are initiated inside a given HTTP/3 connection
   by the client, who sends an extended CONNECT request [RFC8441].  If
   the server accepts the request, a WebTransport session is
   established.  The resulting stream will be further referred to as a
   _CONNECT stream_, and its stream ID is used to uniquely identify a
   given WebTransport session within the connection.  The ID of the
   CONNECT stream that established a given WebTransport session will be
   further referred to as a _Session ID_.

We don't really enforce this in anyway - and our current design likely means we don't even support multiplexing.