Refactor: controller: Rename election_trigger to election_timer

This change makes it clearer that election_timer is of the same type
(fsa_timer_t) as all the other timers.

The name "election_trigger" was helpful for disambiguation versus
"election_timeout", until election_timeout was removed in 97e2b0af.

Ref T239

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>