Implement worker-side components of SyncSGD without backup worker

[hjp615]

Until now, dolphin supports two models for parameter server : asynchronous model and SSP model.
They are faster than synchronous model because speed of synchronous model is limited by the longest training-time worker.

However, the paper Revisiting Distributed Synchronous SGD written by Google Brain team states that SyncSGD with backup worker can solve both convergence problem and speed efficiency problem. So, it would be nice if we implement SyncSGD with backup worker on dolphin and compare them each other.

Implementing worker-side components of SyncSGD without backup worker is a first step to implement SyncSGD with backup worker.