[DOLPHIN-176] Create partitioned, queued, single-node parameter server

[bchocho]

Closes #176

• Adds a new Parameter Server, PartitionedParameterServer. The server supports atomic, in-order processing of push and pull operations, using multiple threads.
• Renames the existing "Single Node" Server as ConcurrentParameterServer.

I've done a simple performance comparison between PartitionedPS and ConcurrentPS by running the unit tests (1million reads + 1million writes) on my local laptop. The PartitionedPS has worse performance. Note that the ConcurrentPS can lose updates.

• ConcurrentPS: Avg 1.5s
• PartitionedPS: Avg 2s

Also, the unit test does not include hash computation. A version of the test (not committed) that uses MurmurHash had an Avg runtime of 3s. However, this test also requires the key to be copied into a byte array, which we avoid in the actual implementation by using the original serialized value from the message.

[jsjason]

I did a pass and everything seems great. The new tests work as intended, and the existing dnn w/ ps example works well too. I'm merging this.