Revert "Add comment about why table.set_triggers drops a trigger"

This reverts commit 983fb73d300b2e56c389a24bcc7bedf519fdcf20.
Revert "Drop varnish-invalidation triggers from user tables to tablemetadata"

This reverts commit e8fd6cc7996ac07c817171e4a4e4a7529d3a259f.
Revert "Create varnish invalidation function _before_ the trigger using it"

This reverts commit c5db6ab89e36056ac52acb8ddb686614b5ccff34.
Revert "Refactor user database triggers"

This reverts commit 9f830270a29365ae019b39695087357b16a90cd9.