Make WriteAll and WriteAllAsync return the number of written rows

[kevin-montrose]

Visible changes:

• WriteAll now returns int instead of void
• WriteAllAsync now returns ValueTask<int> instead of ValueTask

Internal changes:

• In the async paths there's now one more potential async transition, if the final cleanup task has not completed
  • Cesil used to return the cleanup task directly regardless of if it was completed or not