Replace RPC heartbeat by checking if stdin is opened

[guillaumerose]

CRC always issues RPC calls sequentially (each method of machine.Client interface use it's own driver and they don't goroutines). That means we could potentially remove the server and just send json commands on stdin, and read stdout.

There is still one mechanism we need to deal with: the hearbeat. This feature was made in case of a driver a failure. If it hangs, it will kill itself.

We can do the same by checking the status of stdin. Once it is closed, the driver must exit.