Allow multiple bt request streams on an incoming quic connection

[jagerman]

Currently we only use the first stream on the storage server for bt requests, but it can be useful to have more than one stream: that way you could use one for quick polling, and use another for large requests (file uploads/downloads).

Not entirely clear what a good interface would be for this: e.g. we could have the first two streams be btreqstreams unconditionally, or we could have a request endpoint that you call to make the next stream become a btreqstream.