Add a time expiring priority queue

[versionzero]

The purpose of this PR is to add expiring priority queue support. The current implementation only support using time as a method of expiration, but a more general method may be possible.

[tnm]

Thanks, interesting idea!

Could you add some tests? If you'd need to do anything with time, https://github.com/spulec/freezegun can help and I'd probably be ok adding it as a test requirement if needed.