Handle TRUNCATE statements on watched Postgres tables

[bennostein]

          'OLD' is set for DELETE triggers and handled in the ELSE branch here; see https://www.postgresql.org/docs/current/plpgsql-trigger.html 

as for TRUNCATE, it won't invoke row-level triggers like this one -- I'll file an issue for now and look into the best way to handle that going forward. would need another statement-level trigger on the PG side, and some additional logic on the skip side to clear out the collection when that trigger fires.

Originally posted by @bennostein in #783 (comment)