We have mixed up relative/absolute on the trait methods for `from_after`
and `from_older`.

Currently if we mix up height/time absolute timelocks when filtering
policies the result is incorrect.

Add a `timelock` module that includes a single public function for
checking if absolute timelocks are the same unit. Use the new function
to fix a bug in policy filtering.

Currently if we mix up height/time absolute timelocks when filtering
policies the result is incorrect.

Add a bunch of assertions that verify the bug.