vinyl: switch vy_task to vtable
vinyl: switch vy_task to vtable

We need one more operation apart from 'execute'. It is 'complete', which
may be used to finish the task from the scheduler context and perhaps
schedule another task. So let's switch vy_task to vtable. Note,
'complete' is only called if 'execute' finished successfully.

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