raft: integration tests

[SUMUKHA-PK]

We need extensive integration tests on the raft cluster covering all possibilities.
The test must use the underlying cluster package and actually create clusters instead of mocking them.

Following are the scenarios we need to consider.

• Normal raft operation.
• Leader failure.
• Leader recovery.
• Multiple leader problem.
• Network partitions and their recovery.
• Follower join.
• Follower graceful shutdown.
• Mock tests where the non-mocked node is a leader.
• Mock tests where the non-mocked node is a follower.

[SUMUKHA-PK]

We need to add more to the list once we figure it out.

Only when this issue is resolved, #121 will be merged.