Refactor KNSCore::Engine

This change splits the class in 3:
- EngineBase: Does the cache and installation management
- Transaction: Does the actions related to the different entries
- Engine: Maintains API and keeps working like it used to

It also adds a new mechanism for publicly requesting entries through the
ResultsStream, from EngineBase. This allows to have async queries that
don't strictly depend on the Engine's state.