Compare the methods for concurrent model access

[yunseong]

Subissue of #821

We've been working on Trainers that run with multiple threads. When accessing and updating model parameters, three type of synchronization are possible:

1. non-hogwild: shared model with lock
2. hogwild: shared model without lock
3. thread-local: unshared model per thread without lock

Current ModelAccessor implementation supports 3. We should compare all versions to see how one method compels the others and set it as the default implementation.