Do flow control on incoming streams

[fasterthanlime]

Not exactly sure how it should work yet (ie. when and how much capacity to give back to the peer).

Ideally we shouldn't wait until it reaches zero, especially if we've already processed what they've sent us. It depends how fast it's processed by the user on our side I suppose?

We should also reset the stream if we don't care about the request body (acting as a server).

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The reason this is on top of mind is because StreamIncoming::capacity is unused right now, and rustc is unhappy about it.