thread delegate must be destroyed before thread

[iCollin]

Fixes #3387

This PR is ready for review.

Risk

This PR makes no API changes.

Testing Plan

manually as described in issue

Summary

The thread delegate destructor writes on its thread_ pointer, so we must ensure that thread delegates are destroyed before threads.

CLA

• I have signed the CLA

[Jack-Byrne]

Searching for delegate() there are a lot of uses where set_delegate(NULL) is not called unlike the iap destructor example.

BluetoothDeviceScanner::~BluetoothDeviceScanner() {
  thread_->join();
  delete thread_->delegate();
  threads::DeleteThread(thread_);
}

Do these other uses need to call set_delegate(NULL) before calling delete?

[iCollin]

Do these other uses need to call set_delegate(NULL) before calling delete?

They do not, Thread holds a raw pointer to the delegate and will not do anything with the delegate when its destructor is called.

[iCollin]

Please notice that a new solution has been proposed in 30f5e86