Support consistent read with leader lease

[drmingdrmer]

Openraft has a mechanism called leader lease. This mechanism ensures that a follower does not elect itself as a leader before a certain period of time, which is determined by adding the timer_config.leader_lease + timer_config.election_timeout. The follower refreshes the lease by updating the last-updated-time of the vote. However, the current issue is that the leader does not update the lease yet.

To support reading consistent state, openraft needs to :

• The leader should extend its lease when a log is accepted by a quorum.
• and add a public API such as Raft::read(sm: &mut impl RaftStateMachine) that atomically checks leader lease and reads some data from the state machine.

openraft/openraft/src/core/raft_core.rs

Lines 1345 to 1357 in 5415420

	let current_vote = self.engine.state.vote_ref();
	let utime = self.engine.state.vote_last_modified();
	let timer_config = &self.engine.config.timer_config;
	let mut election_timeout = if current_vote.is_committed() {
	timer_config.leader_lease + timer_config.election_timeout
	} else {
	timer_config.election_timeout
	};
	if self.engine.is_there_greater_log() {
	election_timeout += timer_config.smaller_log_timeout;
	}

openraft/openraft/src/engine/handler/vote_handler/mod.rs

Line 124 in 5415420

self.state.vote.update(*self.timer.now(), *vote);

Originally posted by @drmingdrmer in #262 (comment)

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