vinyl: add task accounting to global scheduler statistics

This patch adds the following new fields to box.stat.vinyl():

  scheduler.tasks_inprogress - number of currently running tasks
  scheduler.tasks_completed - number of successfully completed tasks
  scheduler.tasks_failed - number of aborted tasks

tasks_failed can be useful for monitoring disk write errors while
tasks_inprogress and tasks_completed can shed light on worker thread
pool effeciency.