[feature] CacheTTL Store option

[judahrand]

Perhaps nice to offer a slightly more robust in-memory store to use? I say more robust as I assume that one of the concerns with using a simply dictionary is that it could blow up in size memory-size if the number of keys grew out of control. The cachetools.CacheTTL implementation is actually a TLRU implementation so that could be a good thing to use.

https://github.com/tkem/cachetools/