Replace in-memory storage with H2

See thread [1] on the dev mailing list for more context. Ultimately we've found creeping complexity by hand-rolling the in-memory storage schema, and believe that a relational database will make development easier. We've chosen H2 to serve as the in-memory storage engine, and will replace individual Store implementations until all of org.apache.aurora.scheduler.storage.mem is removed.

[1] http://mail-archives.apache.org/mod_mbox/incubator-aurora-dev/201404.mbox/%3CCAGRA8uPKh81F3eCp%2Bf7fE84%2B2m%2ByzounTEcbhNpTaUFWqFt-Xw%40mail.gmail.com%3E