Add Zookeeper 'database' class

Adds a Zookeeper database class, for backing up snapshots and txlogs.  To restore, you can grab the file from cryo, un-tar it (giving you a snapshot.* file and a log.* file), place those files where your zoo.cfg says they should go, and bring up the zookeeper node.  This works even if you bring up a standalone configuration, which is useful if you just want to look at the state of zookeeper as of a few days ago.
Forgive my ignorance, but what happens to these temp directories over time? I don't see an equivalent of the `at_exit { ... }` line in `Utils#get_tempdir`.