vinyl: use heap_top instead of heap_iterator in vy_schedule()
vinyl: use heap_top instead of heap_iterator in vy_schedule()

This is pure refactoring. No functional changes intended.

rtsisyk/tarantool,rtsisyk/tarantool,mejedi/tarantool,mejedi/tarantool,rtsisyk/tarantool,rtsisyk/tarantool,mejedi/tarantool,mejedi/tarantool