Adds stop function so we can stop polling anytime

What does this MR do?

Adds a stop method to poll class so we can stop poll. This is needed in order to use Poll class with visibility lib:https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10181

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

#29569 (closed)