Batch Writer implementation doesn't need to synchronize on addMutation(String, Iterator)

org.apache.accumulo.core.client.impl.TabletServerBatchWriter locks when a mutation is added. It does not need to lock when iterating through an Iterator<Mutation> to add them one at a time. This can have add significant performance issues to the other threads servicing the batch writer's bins if the Iterator that is passed in is poorly performing.