terminology: use "expiration" instead of "expiry" consistently with spec

[nothingmuch]

High-level bikeshed: I just noticed we mix the usage of "expiry" and "expiration" throughout our code. IMO we should just stick to "expiration" since that's the terminology used in BIP77.

Originally posted by @spacebear21 in #1047 (review)

[arminsabouri]

@nothingmuch heads up. I assigned this one over to me