Livecheck: add throttle days parameter

This replaces the `throttle_max_age_days` method in the `Livecheck`
DSL with a `days` parameter on the existing `throttle` method. This is
cleaner and `throttle days: 1` should be readily understandable.
`throttle 5, days: 1` should be interpreted as "every five versions or
one days, whichever comes first" and I've explained this in the docs
comment for the `throttle` method.

This also the removes "max age" terminology in livecheck/livecheck,
simply using "throttle days" and "throttle interval" concepts. These
concepts should work better for general day-based throttling.