Use asyncio.Event for handling state changes

[rapsealk]

Python's asyncio provides several primitive features for synchronization. Among them, the one currently attracts me is asyncio.Event.
There are continuous state changes in Raft and we have to keep eyes on the changes for post-processing. We are using a loop in the current version, but it seems that there could be a better way to implement this process if we utilize asyncio.Event.