cleanup statistics aggregators

@anbangx @JavierJia @Elmira88 
All statistics can now be handled in the compute functions hadoop Counters, like so:

 - `getCounters().findCounter(GraphMutations.Num_RemovedBridges).increment(1);`
 - or equivalently but slower: `getCounters().findCounter("GraphMutations", "Num_RemovedBridges").increment(1);`