prevent tagging existing products when first seen by task

[tekhaus]

This update was suggested by a merchant who noted that when the bulk scan task runs for the first time, the task will tag all existing products that were already in stock.

To prevent this, a "first seen" timestamp metafield value is now added to a product when it is first encountered by the task (in any run mode). New logic was added to prevent adding tags to products unless their "first seen" date is older than their "back in stock" date.

That logic however would prevent newly created products from being tagged, so on product creation events only, the "first seen" metafield value is set to the product creation date. This allows the first back in stock timestamp set on the new product to always be newer, and thus the product would be tagged.