wake up the blocking pipe goroutine

[hmgle]

graftcp/graftcp-local/local.go

Lines 207 to 210 in fc5d1fb

	func pipe(dst, src net.Conn, c chan int64) {
	n, _ := io.Copy(dst, src)
	c <- n
	}

graftcp/graftcp-local/local.go

Lines 198 to 202 in fc5d1fb

	go pipe(conn, destConn, writeChan)
	go pipe(destConn, conn, readChan)
	<-writeChan
	<-readChan
	conn.Close()

will block until all io.Copy return.