Fix normalize-done race causing sync/normalize to execute in parallel after pause/resume

[serprex]

Fixes #1448

Solves via two redundant angles:

1. drain normalize-done at start
2. don't send normalize-done when ctx.Err() != nil

The race was consistently happening when normalize received cancel while in normalize activity. It'd then send normalize-done to parent, who already having returned continue-as-new but waiting on children to finish, would have the signal buffer for its next run