When implementing payment changes (MPP and Trampoline) I found it very confusing that cltv expiry wasn't typed (and that expiry variables were half of the time incorrectly named, often not mentioning that it was a delta).

This change makes it simpler to reason about expiries and let the compiler help us when we're messing things up between expiry delta and absolute expiry.

Apologies for the long and tedious review since it touches most files :D