Decouple SimpleTransientStore implementation from InMemoryCacheImpl

SimpleTransientStore uses InMemoryCacheImpl to store data temporarily but it doesn't really use it as a cache.

However we want to move our Cache API to be a real cache, i.e., with no guarantee of data being retained at all.

So we must decouple that transient store implementation from the Cache APIs and just store the data in-memory using an ad hoc Guava cache.