Use environment, reserved client, and target client id

This patch fixes the steps in Handle Fetch that
 - Get various clients' info and expose them to fetch event handlers
 - Match/attach a controller, if available, to the given environment
by referencing the request's reserved client and target client id.

Related commits:
 - HTML: whatwg/html@2b93f9e
 - Fetch: whatwg/fetch@fb87b70

NOTE: Changing the methods of Clients/Client will be done as a
separate patch.