Make Raft groups quiet when idle

[sciascid]

This patch adds the ability to make Raft groups quiescent, so that leaders stop sending heartbeats after a long idle time. It works as follows:

• When a leader detects a long idle time it sends a Quiesce message to its followers, and stops sending heartbeats.
• Upon receiving a Quiesce message, followers no longer expect heartbeats from the leader, by stopping their election timers.
• A leader or follower unquiesces upon receiving any kind of request: a new proposal, append entries, vote requests, and so on.

Potential advantages and uses cases:

• Fewer messages over the network and less ticking in deployments with a large number of inactive Raft groups
• Read-only streams

Disadvantes:

• Failures may go unnoticed for a long time (Could be avoided if Raft layer could be notified of dropped connections?)
• The first request to a quiesced group may experience higher latency should nodes find out that they need to go through a new election efore proposal can be pushed forward.

This is a proof of concept, it's incomplete and meant for experimentation only.

Signed-off-by: Daniele Sciascia daniele@nats.io