Disk-backed object cache

Rationale
A supplement to memcache-based object cache, which will allow us to greatly increase the amount of space dedicated to caching parsed pages.

After an initial attempt to deploy Ehcache, we switched to a different strategy of deploying a custom MySQL-based solution.

Additional documents

 * User requirements:
 * Specifications:
 * Software design document:
 * Test plan:
 * Documentation plan:
 * User interface design docs:
 * Schedule:
 * Task management:
 * Release management plan:
 * Communications plan:
 * Status updates