vinyl: review fixes for the new scheduler
vinyl: review fixes for the new scheduler

* start worker threads from scheduler thread
* rename vy_workers_start()/stop() to vy_scheduler_start()/stop()
* streamline scheduler and worker loops
* remove `return 1` from vy_plan()
* move checkpoint code to vy_wait_checkpoint()

Fixes #1631

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