Decouple computation and communication by introducing worker-side model cache

[wynot12]

In Dolphin, workers always pull model from servers in synchronous manner.
Computation stalls until the model pull finishes.
It wastes much of worker resources and slows down overall progress.

We need to decouple computation from communication by introducing worker-side model cache.
Then computation can keep going regardless of communication.

The model cache will be refreshed in background.
As a first try, we can update cache in best-effort manner.
Later, we may support SSP.

[yunseong]

Part of #803

Let's separate out the issues about policies for refreshing cached objects.