Blender Git Loki

Depsgraph: Replace LIB_TAG_DOIT with hash lookup

This allows us to:

- Not mock around with tags stored in a global space,
and not to iterate over all datablocks in the database
to clear the tags.

- Properly deal with datablocks which might not be in main database.

While it sounds crazy, it might be handy when dealing with preview,
or some partial scene updates, such as motion paths.

- Avoids majority of places where depsgraph construction needed bmain.

This is something what could help in blender2.8 branch.

From tests with production file here did not see any measurable slowdown.

Hopefully, there is no functional changes :)