order of operations fix in the duration_to_next_slot calculation (#981)
order of operations fix in the duration_to_next_slot calculation (#981)

* order of operations fix in the duration_to_next_slot calculation

* use slot_clock.duration_to_slot() to simplify the duration_to_subscribe calculation

* fix error message
sigp/lighthouse,sigp/lighthouse,sigp/lighthouse,sigp/lighthouse