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

Pull request for #8 

Created TimestampTracker to keep track of timestamps in Zookeeper.  Modified GC iterator to get oldest active timstamp from Zookeeper. Created integration tests for TimestampTracker and GCIterator
synchronizing on zookeeper I/O here will cause foreground threads trying to get a timestamp to unnecessarily  block.  Should be able to make this update concurrently w/ foreground threads requesting a timestamp.