Rewrite (incrementally or in full) the ContributionTracking modules. This work is complete when we no longer have a SPOF in contribution tracking data.

• Ideally we consolidate to a single PHP library.
• Schema migration must be deployed ever so carefully, and with no irreversible steps until we're 100% confident in the new data. This is everything we own, don't set it alight.
• Push on timeline for Redis 3 clustering support, which allows for invisible high availability with no added code complexity. See Redis 2 HA hooks in DI::DonationQueue to see why this is important.
• Shared sequence generator.
• Entries older than 1 hour can be kept in cold storage.
• We'll need a staging Redis cluster for QA.
• Vagrant module to easily provision a 2+ -node Redis cluster.