Do not run a "table exists" query for every migrated row of data (regardless of whether it's a metatag row or not!)

Problem/Motivation

metatag_migrate_prepare_row() runs a "table exists" query for 100% of migrated rows, even if it is a row that has absolutely nothing to do with Metatag! 😅🤯

This was introduced in #2563649: Migrations: Metatag-D7 basic entities.

Steps to reproduce

Proposed resolution

Statically cache the result, so it is run only once per request.

Remaining tasks

User interface changes

API changes

Data model changes