Implement an in memory cache.

[Sutto]

This is a WIP PR, but it's worth nothing this implements a simple, shared in memory cache.

The default file cache has a pretty serious memory leak in pull mode - Since it instantiates and loads the cache for each policy, memory grows a copy of the cache for each policy.

This uses a single shared dict, and a simpler structure, to avoid that when want a cache for the lifetime of the run but not to be persisted.

[kapilt]

sounds good re mem cache, although plus test. though that raises a bigger question of whats causing the extant cache to stay resident.

[kapilt]

extant file cache leak fix in #1281

[kapilt]

lgtm, thanks