ws: make Dispatcher abstract

The `Dispatcher` type is now opaque and not exported by
`.AsyncUpdates.Dispatcher`, and it has been moved out of the `.Types`.

This ensures that we can make the internal record fields private, and
offer accessors for things like `terminateDispatcher`. This preserve
information hiding and allows us to change the internal way of
terminating a dispatcher (for example switching away from normal
`forkIO` in favour of `async` & co) while not breaking client's code.