How i can get a Non Objetion Letter?
You ask your sponsor for it, you don't need to finish your contract before asking for it.

what is it exactly?
It's just says that he approves a transfer of visa, or you coming back to the country under another sponsor in less than 2 years.

Can my sponsor delete my visa?
Yes.

Can he send me out of Qatar when i m asking for changing?
Yes.

Is it possible that my old and my new sponsor are making something like a hired-out worker contract?
Yes. But I think it's for a limited time (maybe 6 months?)