tor-memquota: Provide StreamUnobtrusivePeeker

My memquota-using queue needs this, so it can peek at the age of the oldest item in the queue, without getting involved in async task stuff. futures::stream::Peekable won't do because it only has async peek methods, which the memquota system can't/mustn't call.

I'm not particularly pleased with this name. We need names for both the type, and the trait we'll almost certainly want to introduce in the future.

Part of #351