reduce rollback-to-stable work for fast-truncate pages

Rollback-to-stable rolls back instantiated fast-truncate pages twice, once when processing the parent internal page and a second time when processing the child leaf page.

I don't see a problem in the current code, but it's dangerous because rollback-to-stable uses the WT_PAGE_DELETED.durable_timestamp value to determine if rollback is needed. When a page is instantiated, the WT_PAGE_DELETED.durable_timestamp moves from the WT_PAGE_DELETED structure to the individual WT_UPDATE structures, so after instantiation rollback-to-stable is taking the value from the wrong source.

Rollback-to-stable should only check WT_PAGE_DELETED.durable_timestamp for pages that have not yet been instantiated.