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/000022_create_logs_table.up.sql

@@ -0,0 +1,5 @@
+CREATE TABLE IF NOT EXISTS logs(
+   id bigserial PRIMARY KEY,
+   task_id bigint REFERENCES tasks(id) NOT NULL,
+   message text
+);