fix(mint): drop queued chunks when cancelling requests

[arrangabriel]

Drop queued body chunks when a request's stream ends

process_response/2 pops a finished request's ref from state.requests, and Mint drops it from the conn struct, but remaining chunks are still present in request_stream_queue.
handle_continue(:process_request_stream_queue, ...) then drains the queue and calls Mint.HTTP2.get_window_size/2 on the stale ref, which raises and kills the connection process.

Fixes #594

Changes

• drop_request/2 pairs State.pop_ref/2 with drop_queued_request_chunks/2, and it is now called every time a single request is dropped.