Use a flag to track updates/deletes in SQL visitor

[leboshi]

This was a small refactor we did to remove guesswork. Especially collector.value.include?(' UPDATE ') is prone to false positives, for example if you're working with an error column that logs query info, like SELECT * FROM errors WHERE message ILIKE '%query failed: UPDATE table%'. It also just felt weird to read from the collector to try to figure out what to do next.