NRG (2.11): Vote request cancels catchup, new leader could have rejected subsequent catchup messages

[MauriceVanVeen]

Antithesis triggered a case where:

• a follower (S0) is behind and triggers catchup from the leader (S2)
• another follower (S1) triggers leader election
• the follower (S0) allows follower (S1) to become leader
• the leader (S2) sends a new message, which new leader (S1) rejects because it's on a higher term
• the follower (S0) now receives above message that new leader (S1) rejected, providing previous leader (S2) with quorum
• the new leader (S1) rejected that message, so previous leader (S2) has now desynced since it applied the message

This is solved by canceling catchup once a leader election happens for a higher term. The new leader will then resume our catchup once it's elected.

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