Rename voting_schedule_*() functions.

This is an automated commit, generated by this command:

./scripts/maint/rename_c_identifier.py \
        voting_schedule_recalculate_timing dirauth_sched_recalculate_timing \
        voting_schedule_get_start_of_next_interval voting_sched_get_start_of_interval_after \
        voting_schedule_get_next_valid_after_time dirauth_sched_get_next_valid_after_time