* The code should be made cache-aware and use the Requests-Cache `CachedSession` sessions.
This allows for more granular control over where in the code cache is (not) used.
* Downloaded thumbnails are cached (by us) in `$CACHE/thumbnails/node_uuid/filename`.
* Use a non-cached HTTP GET to download these (using [session.cache_disabled()](http://requests-cache.readthedocs.org/en/stable/api.html#requests_cache.core.CachedSession.cache_disabled)).
* HTTP headers are stored as JSON key/value pairs in `$CACHE/thumbnails/node_uuid/filename.headers`.
* Use the ETag and If-Modified-Since headers to prevent unnecessary re-downloading.
* Check Content-Length header against actual file size to detect partially-downloaded files that need re-downloading.
* Downloaded files (such as textures) are handled in the same way as thumbnails (described above),
but have their metadata stored somewhere else (described below).
### Filesystem Layout
* Top-level texture directory, referred to as `$TEXTURES` in this document, can be configured per
Blender scene. It defaults to a global addon preference, which in turns defaults to `//textures`.
* Texture files are stored in `$TEXTURES/{project_name}/{node_name}/{node_name}/.../{file_variant}-{file_name}`.
`file_variant` is "col" for colour/diffuse textures, "nor" for normal/bump textures, etc.
* Metadata is stored in `$TEXTURES/.blender_cloud`, with the following subdirectories:
*`projects/{project_uuid}.json` containing the project document from Pillar
*`nodes/{node_uuid}.json` containing the node document from Pillar
*`files/{file_uuid}.json` containing the file document from Pillar
*`index.sqlite` containing a mapping from `{project_name}/{node_name}/{node_name}/...`
to the UUID of the last-named path component (project, node, or file). This allows us
to map the path of a filesystem path to its Pillar document.
