ClientProtocol.isCancelled: the transport consults this through the
ClientProtocol seam to drop a late response for a request the client has
cancelled (basic/utilities/cancellation). The id is evicted from the
cancelled-id set once its late response has been observed and dropped here,
so the set does not grow without bound over a long-lived client; an id
whose late response never arrives is reclaimed by the size backstop in
cancel.
ClientProtocol.isCancelled: the transport consults this through the ClientProtocol seam to drop a late response for a request the client has cancelled (basic/utilities/cancellation). The id is evicted from the cancelled-id set once its late response has been observed and dropped here, so the set does not grow without bound over a long-lived client; an id whose late response never arrives is reclaimed by the size backstop in cancel.