Prevent slow turbo frame requests from resetting cookies

[domchristie]

This pull request fixes an issue where slow turbo-frame requests can inadvertently reset cookies.

For example, let's say we have a form that changes the session and a <turbo-frame> that loads a slow endpoint:

<form method="post" action="/session">
  <button type="submit">Change Session</button>
</form>

<turbo-frame id="slow" src="/slow"></turbo-frame>

Here's a potential flow:

1. Page loads, <turbo-frame> starts loading
2. User taps "Change Session"
3. That request completes and the session cookie is updated
4. <turbo-frame> request completes later and resets the session cookie to what it was in step 1.

Reproduction: https://github.com/domchristie/turbo_frame_cookies_test

The fix is to cancel the frame's current fetch request when the <turbo-frame> disconnects.

[domchristie]

Recursive <turbo-frames> look like they rely on the current behaviour. I've updated the disconnect flow to account for this, but it's not the prettiest.