Adds option to flush cache after ttl

[amarBitMan]

No description provided.

[srijan]

This does not make sense. We don't want to flush if we're trying to get the key.

We only want to flush if timer is expired and no one wants the key anymore.

[srijan]

We're actually using a fork from the 0.1 tag of the original code to keep it simple. This version does not have expiry logic.

Check the master branch to see how to expire. That version runs a separate expirer gen_server which periodically prunes the expired entries.