Convert Behavior + BehaviorRegistry to use config instead of settings.

Refs #2298
Consolidate Cache classes to use _config instead of settings.

This also makes the properties protected instead of public, which feels
like a net win to me.

Refs #2298
Remove lies in doc blocks.
Re-add accidentally removed method.
Merge pull request #2326 from markstory/3.0-config-vars

3.0 Convert Cache + Behavior classes to use _config