What other mechanisms have you compared it to?

To a lack of any.

How do you define "pain" in a general case?

Sharp negative reinforcement in a behavioristic learning process.

How does one define unnecessary pain?

Useless/inefficient for the necessary learning purposes.

Does boredom counts as a necessary pain?

Depends on the circumstances. When boredom is inevitable and there's nothing I can do about it, I would prefer to be without it.

How far in the future do you have to trace the consequences before deciding that a certain discomfort is unnecessary?

Same time range in which my utility function operates.

(EDIT: I'm sorry, I should have asked you for your own answers to your questions first. Stupid me.)