Form builder cache can overflow with a moderately large webform

In Drupal Gardens we saw someone use form builder to make a webform that was big enough to cause the {form_builder_cache} table to overflow, leading to fatal errors.

The form itself was large, but not gigantic - maybe 30 fields or so.

Drupal core uses 'longblob' columns (rather than 'blob') for its cache tables, which pretty much guarantees they will never fill up. The attached patch switches Form Builder to use 'longblob' columns also.