Fixes #16699 and #18960 by ensuring that we release our HTTP
`spawn_local` tasks when the HTTP resource is dropped.
Because our cancel handle was being projected from the resource via
`RcMap`, the resource was never `Drop`ped. By splitting the handle out
into its own `Rc`, we can avoid keeping the resource alive and let it
drop to cancel everything.
This commit changes "ProcState" to store "file_fetcher" field in an "Arc",
allowing it to be preserved between restarts and thus keeping the state
alive between the restarts. File watchers for "deno test" and "deno bench"
now reset "ProcState" between restarts.
