Introduce a serializing key-value cache proxy

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