Summary:
Ref T11954. I want to store some lists/arrays in the mutable (database) cache, but it only supports string storage.
Provide a serializing wrapper which flattens when values are written and expands them when they're read.
Test Plan: Used by D16997. See that revision.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T11954
Differential Revision: https://secure.phabricator.com/D16999
