NRG: Success signaling & stepdown during catchup

[MauriceVanVeen]

This PR fixes some potential issues with our Raft replication:

• Could technically do a divide by zero in n.Applied(index), now guarding against that. As well as recalculating n.bytes after n.truncateWAL(term, index).
• Grab ae.lterm from the append entry earlier, and stepdown from leader or candidate if a higher term is detected during catchup.
• Handle case where catchup is canceled but we still receive an entry from a previous catchup, in which case we ignore that entry.
• Don't signal success/non-success after a mismatch. This could result in wrongfully providing quorum when having an outdated log.

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

[derekcollison]

LGTM

[neilalexander]

LGTM