There are a few ways for `connect()` to finish:
- `Browser.close()` from the client side.
- Browser on the server side did exit (e.g. crashed).
- Connection was dropped by either of the sides.
We reduce all the cases to the last one by dropping the
connection when client wants calls `Browser.close()` or
server-side browser exits.
In all these cases we should properly cleanup on the server side,
and ensure that all promises reject on the client side.
This makes it much nicer to use `BrowserType` because it no longer has a template.
Technically a breaking change because of the rare edge case where someone used their own non-browser type inside the template, but I don't consider that intended behavior and think this is fine.
