Optimized BSONComparator's compare

Enron:
    With MailPair: 62.53s
    With old BSONComparator: 63.10s
    With new BSONComparator: 62.98s

PageRank (79,737,450 records into 36,861,697 groups):
    With old BSONComparator: 823779 milliseconds = 13.7296 minutes
    With new BSONComparator: 614995 milliseconds = 10.24 minutes