Closes #8 - Garbage collection iterator now only clears data prior
to oldest active snapshot.  Created TimestampTracker to keep track
of timestamps in Zookeeper.  Modified GC iterator to get oldest
active timestamp from Zookeeper.  Snapshots and transactions now
have resources that need to be closed.
Merge pull request #157 from mikewalch/fluo-8

Closes #8 - Made garbage collection iterator determine oldest active snapshot.