Switch lookup table cache implementation to support per-key TTL

To properly support data adapters that can indicate expiration times, such as DNS, HTTP cache headers etc, our in-process cache should natively support per-key TTL eviction, too.

It would in theory be possible to build it on top of Guava, but that would neither be efficient nor trivial to build.

One obvious candidate to switch to is https://github.com/ben-manes/caffeine