Use Terminado as backend terminal server handler

[andfoy]

Now that @blink1073 has extended Terminado to use pywinpty with the newest high level API, I think we can use that server instead of our own in-house server terminal management routines based on pywinpty's low level API.

What do you think @ccordoba12 ?

[ccordoba12]

Sure, I think it's better to depend on Terminado instead of our own server.