Cancel HTTP/2 streams when requests are interrupted

[ekmartin]

Interrupted HTTP/2 requests leave their streams open after the connection is released back to the pool. These abandoned streams consume the peer’s concurrent-stream capacity until the peer completes them, potentially causing healthy requests to fail with GoawayError: Exceeded maximum concurrent streams.

This change cancels an abandoned stream before the interruption propagates:

• Use ensure and a completed flag around write_request and read_response, covering both timeouts and Async::Stop.
• Send RST_STREAM with CANCEL only when the request is incomplete, the stream is active, and the connection is open.
• Preserve the existing RefusedError checks for GOAWAY and closed connections.

A new loopback-server regression test times out /slow, verifies that the stream registry is empty, and successfully requests /fast on the same connection.