[IMPROVED] NRG: Reliable peer current/lag state

[MauriceVanVeen]

Follow-up to #7402. Also resetting the last replicated index, since we only know it if we're the leader, since only the leader is in contact with all servers and knows the amount of messages they have persisted in their logs.

The Lag and Current values were mostly unusable/stale/incorrect when read on a follower node. These values are returned in requests like stream and consumer info, as well as JSZ. So, this PR makes the n.Peers() state more usable and consistent:

• The Lag/Current fields now accurately describe whether the current server is current (or lags) when compared with the peer in the list.
• Only the leader will report Lag. All followers that are part of quorum will report Current: true, Lag: 0. Other non-current followers not part of quorum will show Lag of the amount of entries that are committed but not yet persisted on this peer. (This was already the case)
• A follower will always report other followers as not current with no lag. It doesn't have any contact with other peers, so this data is not useful either way. (Previously this contained stale/unused data making it incorrect and deceiving at worst).
• A follower will report in Current whether it has seen the leader recently.

Signed-off-by: Maurice van Veen github@mauricevanveen.com

[sciascid]

=== RUN   TestJetStreamLeafNodeClusterExtensionWithSystemAccount/true-true
    jetstream_leafnode_test.go:532: Expected a cluster peer count of 4, got 2

A regression? Or just flaky test?

[MauriceVanVeen]

Or just flaky test?

Flaky

[neilalexander]

LGTM