This commit adds "CancelHandle" to "ConnResource" and changes
"op_http_next_request" to await for the cancel signal. In turn
when async iterating over "Deno.HttpConn" the iterator breaks
upon closing of the resource.
This commit fixes the URL returned from `request.url` in the HTTP server
to be fully qualified. This previously existed, but was removed and
accidentially not readded during optimizations of the HTTP ops.
Returning a non fully qualified URL from `Request#url` is not spec
compliant.
