vinyl: factor out scheduler checkpoint functions

Introduce the following functions:

  vy_scheduler_begin_checkpoint()
  vy_scheduler_wait_checkpoint()
  vy_scheduler_end_checkpoint()

and make the Vinyl API methods use them instead of playing with the
scheduler directly.

Rationale:
 - It will help move the scheduler to a separate file in future.
 - We want to avoid waking the scheduler on checkpoint if Vinyl is not
   used, because the scheduler starts worker threads on the first
   wakeup. At the same time, we have to call vy_log_rotate() on
   checkpoint no matter if Vinyl is used or not (it needs to update its
   internal housekeeping). Factoring out the scheduler functions will
   allow to achieve this goal neatly.