WIP Path Cache Core v0.15 In preparation for Technical and Functional QA

Adding 0.15 Release notes here awaiting merging into master and proper release:

v0.15.0
Improved folder synchronization and local caching. **This is a larger update and we strongly recommend testing prior to studio rollout.**

**Details and highlights:**

**Shotgun Path Cache**
- The v0.15 release introduces a stronger and more robust folder creation mechanism where the associations between folders on disk and entities in shotgun are tracked in shotgun rather than in a local file on disk (the so called path cache file).
- On application launch and folder creation, the locally cached folder database (the path cache) will be synced against the list of folders in Shotgun.
- On setups where several local independent storages are used, it is now possible to "replay" folder creation on each local machine via the path cache synchronization mechanism.



**Cache Location**
- The path cache file no longer exists in the primary project storage but instead resides on a user's local machine (inside `~/.shotgun` on Linux, in `%APPDATA%\Shotgun` on Windows and in `~/Library/Application Support/Shotgun` on Macosx. 
- Other cache entries have also been moved into this location - the `cache_location` property in the Engine/Framework/App baseclass no longer points into the cache folder in the Pipeline Configuation but rather to the local folder mentioned above. This improves performance on setups with slow networked file storage.
- The cache location is configurable per data type and can be overridden via a core hook if desirable.



minor but should we use sgtk for new hooks?