Honor context deadline in send operations to prevent indefinite hangs

[novahe]

Description

fix #174
where ttrpc client send operations (Write/Flush) could hang indefinitely if the peer stopped reading, even when the request context had expired.

Changes

1. Context-Aware Send Path: Updated Channel.send, Client.send, serverConn.run, and stream.send to accept and propagate context.Context.
2. Write Deadlines & Cancellation:
   • Implemented net.Conn.SetWriteDeadline in channel.send based on the context's deadline.
   • Added a watcher goroutine in channel.send to support manual context cancellation (ctx.Done()), ensuring that context.WithCancel also interrupts blocked I/O.
3. Robust Connection Management:
   • Introduced failSend to ensure that any write-side failure (including timeouts and cancellations) results in closing the underlying connection. This prevents protocol corruption due to partial bufio writes.
   • Added mapWriteTimeout to correctly translate I/O timeout errors into context.DeadlineExceeded or context.Canceled.
4. Stream Cleanup: Updated createStreamWithContext to ensure that streams are properly removed from the client's internal map if the initial send fails.

[novahe]

PTAL @dmcgowan @ningmingxiao @samuelkarp