Fix logic race with Context.watchStop
Fix logic race with Context.watchStop

Always wait for watchStop to return during context.walk.

Context.walk would often complete immediately after sending the close
signal to watchStop, which would in turn call the deferred releaseRun
cancelling the runContext.

Without any synchronization points after the select statement in
watchStop, that goroutine was not guaranteed to be scheduled
immediately, and in fact it often didn't continue until after the
runContext was canceled. This in turn left the select statement with
multiple successful cases, and half the time it would chose to Stop the
providers.

Stopping the providers after the walk of course didn't cause any
immediate failures, but if there was another walk performed, the
provider StopContext would no longer be valid and could cause
cancellation errors in the provider.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk