RAFT: RequestTable needs to be populated when becoming leader

[belaban]

When we have 2 requests (3-4) in the leader's RequestTable with last_applied=4 and commit_index=2, and the leader crashes and is restarted (or a new leader is started), then RequestTable needs to be populated with pending requests.
If this is not done, the new leader will start trying to commit pending entries 3 and 4 and when the AppendEntries responses are received, and the RequestTable is empty, we won't ever commit entries 3 and 4.
Solution: when becoming leader, populate RequestTable from the log: add entries in range [last_applied+1 .. commit_index].

[belaban]

Done in Leader.init() -> RAFT.createRequestTable()