V3 comment form timeout-or-duplicate error

When the user takes more than 2 minutes to read the blog post and compose comments, recaptcha verification will return a ‘timeout-or-duplicate’ error.

To reproduce:
1) Enable the plugin for the comment form.
2) Load a post with comment form. Wait more than 2 minutes. Submit the comments.
3) Receive error on comment submission.

Tested on V3. May apply to V2 invisible as well. Suggest getting the recaptcha token right before form submission, not on page load.

See also: https://github.com/google/recaptcha/issues/302