[WIP] Simple Example of Async SGD

Based on what we have discussed, I think it is of great interest to show the following snippet of code.
- Assume features are mapped consecutively(i.e. no Localizer is neede)
- Assume all the node take its own partition of the data(i.e. no scheduler is needed)

Write a simple async SGD that defines ServerHandle and Worker Logic and that is all you need to do. This will demonstrate the core functionality of ps-lite without getting distracted in the other optimizations. And I think the efficiency should be fine for the case we described.

In the long run, we will want to move some of the specific optimization module(Localizer, WorkLoadPool) to dmlc-core eventually to design a user friendly API for more users to use them