Fixes

[mauropasse]

Fixes

• Make sure we exit spin() before destroying members of EventsExecutor, during ~EventsExecutor:
  Add TimersManager::is_running() / Exit spin() before destroying stuff
• Unset gc's callback on ~EventsExecutorNotifyWaitable()

[mauropasse]

Closed since similar to what discussed in ros2#2218 (comment) :

"...The problem is a race condition and I think that the already existing executor->is_spinning() method is the correct way to synchronize and avoid it."

By means of the same approach, the issues I found are gone.

I opened a PR for the only fix that I think it must still be included: #121