Add support for local audience caching.

[ydennisy]

Summary

EdgeKit audience definitions can also arrive from the server, this means to lessen the burden on servers we can cache definitions locally, and just let the server know which audience definitions we have stored locally.

Requirements

• audience definitions should contain a flag, if they are to be cached locally
• edgekit would first check local cache, and send along some meta to the server
• server can reply with any new audience definitions or return an empty response
• the rest of the evaluation would run as normal.