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
This call is made in a sync block.  Want to avoid serializing access to Oracle client.  If 20 threads request a timestamp at around same time, then Oracle client should batch those into one RPC request to oracle for timestamp.