implemented by purely unbuffered channel

TODO List:

• Lab 1: MapReduce
• Lab 2: Raft
  • Part 2A
  • Part 2B
  • Part 2C
  • Part 2D
• Lab 3: Fault-tolerant Key/Value Service
  • Part 3A
  • Part 3B
• Lab 4: Sharded Key/Value Service
  • Part 4A
  • Part 4B (pass 5000 times)
    • Challenge1 (pass 5000 times)
    • Challenge2 (pass 5000 times)