Implement central cache

[cohix]

With the move to a Sat-based architecture, there needs to be a central cache implementation that can act as the default instead of the in-memory store.

The main DeltaV server should hold the cache, and endpoints should be added that the individual Sat instances can communicate with to perform cache operations. This should either replace or be made an alternative to the in-memory store, which is pretty useless when using Sat in a DeltaV system.