New files implementing an asynchronous cache.
Updated docs on example script
Fixed spelling error.  Removed unecessary code
Updated docs
Made changes in response to PR feedback
Use a more specific class for the caching client
Type 'callable' should include a string
Improve the method docs
Update method signatures and docs.  Simplify logic
Merge pull request #5045 from Wikia/AsyncCache

New files implementing an asynchronous cache.