Move logs from array to dedicated table

This avoids the append problem which bloats the database to 200+ gb in a
month and shrinks again to 5g with a full vacuum

migrations/000024_migrate_logs.up.sql

@@ -0,0 +1,3 @@
+INSERT INTO logs (task_id, message)
+SELECT id as task_id, l as message
+FROM tasks, unnest(log) l ORDER BY task_id ASC;