Use larger stream window

[edsko]

Since kazu-yamamoto/http2#130 we no longer have a limited number of workers, but instead have one thread per stream. This means that the warning in network-control

If you use a smaller connection window size, you must ensure that if you are handling fewer concurrent streams than allowed by defaultMaxStreams, that the unhandled streams cannot exhaust the connection window, or risk the entire system deadlocking.

no longer applies (the emphasized if is no longer true). We should therefore set the stream window size to something larger (256 kB?), with a smaller connection window (2 MB?). This should result in fewer WINDOW_UPDATE frames sent.

We should verify this reasoning and check that this does not ensure deadlock.

While we're at it: the numberOfWorkers parameter of http2, mirrored as settingsNumberOfWorkers in http2-tls are both redundant. We should submit PRs to remove them.