tor-proto: add PaddingState channel padding logic

This implements a set of completely isolated logic to figure out what the channel padding state should be, based on some input variables (PaddingVariables) and a boolean indicating whether the channel's usage would imply padding should be used.

The fact that the logic is essentially a pure function from padding variables to padding behaviour makes it very easy to unit-test, as well as embed inside the channel reactor to make padding decisions.

This duplicates the functionality implemented in !657 (merged), as part of a gradual effort to refactor it.

cc @Diziet