Changes

• Propagate Buffer::cancel_on_shutdown() to the created service
• Improve API documentation for channels, timers, futures, and services
• Align service timeout ordering with time::timeout()
• Remove unnecessary Default bound from condition::Waiter future
• Wake pending mpsc receive operations when the receiver is closed
• Recompute bstream sender readiness when its buffer threshold changes
• Saturate overflowing Millis and Seconds conversions
• Remove Clone from OneRequestService to preserve its single-call guarantee