MM-43045: minimize JOIN Posts (#19934)

db/migrations/postgres/000081_threads_deleteat.down.sql

@@ -0,0 +1 @@
+ALTER TABLE threads DROP COLUMN IF EXISTS deleteat;

db/migrations/postgres/000081_threads_deleteat.up.sql

@@ -0,0 +1,2 @@
+ALTER TABLE threads ADD COLUMN IF NOT EXISTS deleteat bigint;
+UPDATE threads SET deleteat = posts.deleteat FROM posts WHERE threads.deleteat IS NULL AND posts.id = threads.postid;