Retaining build_id in ajax_get_form() conflicts with page cache.

I've just run into #1808626: Webform ajax is incompatible with the page cache. - further investigation revealed that the underlying cause is that the build_id is retained in ajax_get_form().

In combination with the anonymous page cache retaining the build_id is a very bad idea. I think it's best to demonstrate this with an example: Two anonymous users fill-out a multi-page form with ajax submission (just like in #1808626: Webform ajax is incompatible with the page cache.).

1. User1 loads the page with the form. A build_id is generated and put into a hidden field. The form_state is stored into cache_form.
2. User1 submits the first page of the form. The values are stored into the very same form_state array in the cache_form-table
3. User2 loads the page with the form and gets the identical build_id from the cache.
4. User2 submits the first page and thus overrides the value that were previously stored for User1.

Effectively this makes it impossible to use AJAX for multi-page forms because it breaks the assumption that different users get independent copies of the form_state array.

Solution strategies

I see two solutions for this issue:

1. Don't retain the build_id just like it's done for non-ajax form submissions in drupal_get_form().
2. Don't display a hidden form-element for the build_id if $conf['cache'] == TRUE and the form is only rendered (as opposed to submitted). ajax_get_form() would need to be adjusted