Create partitioned, queued, single-node parameter server

[beomyeol]

We should improve a single node parameter server. The high-level design will consist of partitions based on the hash of the key, with a queue, kvStore, and thread per partition.

The design is meant to improve on some of the issues with the current single-node parameter server.

• It will not miss updates (#152)
• It will decouple PS work from the NCS threads (#157)

This issue does not cover the client (Worker) side.