a cancelled call does not leave its conn for the next one (V-638)

The watchdog and the end of the call race by construction: a cancellation
landing as the reply arrives closes a conn the call had already finished
with, and c.conn still pointed at the closed socket.

It was survivable before this. A write to a closed socket is
errWriteLost, which re-dials and retries, and that retry is safe because
nothing was sent. So this buys one round trip, not a correctness fix, and
the new test says so rather than pretending to catch a break.

internal/ipc/client.go

@@ -255,8 +255,18 @@ func (c *Client) roundtrip(ctx context.Context, m Method, raw json.RawMessage, r
 	}
 	defer conn.SetDeadline(time.Time{})
 
+	// The watchdog and the end of the call race by construction: a cancellation
+	// landing just as the reply arrives can close a conn this call is already
+	// done with, and c.conn would still point at the closed socket. So a call
+	// whose context ended does not leave the conn behind for the next one,
+	// whichever of the two got there first.
 	done := make(chan struct{})
-	defer close(done)
+	defer func() {
+		close(done)
+		if ctx.Err() != nil {
+			c.drop()
+		}
+	}()
 	go func() {
 		select {
 		case <-ctx.Done():