[Distributed] Add the ability to persist and maintain currentterm for the raft node

This is the quote from raft's paper.

Raft servers must persist enough information to stable storage to survive server restarts safely. In particular, each server persists its current term and vote; this is necessary to prevent the server from voting twice in the same term or replacing log entries from a newer leader with those from a deposed leader. Each server also persists new log entries before they are counted towards the entries’ commitment; this prevents committed entries from being lost or “uncommitted” when servers restart.

Therefore, the persistence of term is important to ensure the security of the system.

At present, because the log persistence has been done but not integrated, and the term persistence is also very necessary, so it will be the work of this issue to integrate the log persistence in the first place and implement the term persistence.