Introduce policies for managing worker-side cache

[yunseong]

We are introducing worker-side cache in #1251 (which was raised #803), however, we currently refresh the cached values in a naive way; we fetch new parameters periodically (e.g., every 10 seconds).

We need to implement the policies and one potential solution in our mind is SSP, where the parameters are refreshed according to the staleness bound (wei et al, SoCC'15, PDF).