[CAY-1139, 1148] Implements Pregel on ET

[JunhoeKim]

Closes #1139 #1148 .
This PR implements a simple version of Pregel on ET. The current version has an environment that only runs Pagerank app. And optimization problems related to graph processing will be covered in future issues.

This program works in the following way:

1. Set the init state

• PregelLauncher submits PregelDriver according to the specified configuration.
• PregelDriver submits PregelWorkerTask to executors. Then all executors starts graph processing app.

2. Communication with master and workers

• In a single superstep, PregelMaster communicates with WorkerMsgManager in each executor using CentComm services.
• SuperstepControlMsg and SuperstepResultMsg determine the termination of current app.

3. Computation and tables

• At every superstep, the Computation interface which are specified by app works around the vertices assigned to the executor.
• ET has a vertex table and two message tables for current and next superstep. These message tables are supervised by MessageManager.

[wynot12]

@JunhoeKim Overall it looks great :)
I've made some minor commits.
Could you take a look?
Thanks!

[JunhoeKim]

@wynot12 It looks good to me. Thank you for refining works!

[wynot12]

@JunhoeKim Thanks for your work :)
I'll merge it.