We signal provider cancellation by hangning a goroutine off of the plan
executor's parent context. To ensure clean shutdown, this goroutine also
listens on a channel that closes once the plan has finished executing.
Unfortunately, we were closing this channel too early, and the close was
racing with the cancellation signal. These changes ensure that the
channel closes after the plan has fully completed.
Fixes#1906.
Fixespulumi/pulumi-kubernetes#185.
4a550e308f