Turn a function's trigger on the branch on or off

A `table` trigger is installed in the branch's database by the worker (`pending_install`, then `installed` or `failed`); each committed matching row change is then POSTed to the function's `fetch` handler with header `chelabase-event: db` and body `{id, type, schema, table, record, old_record, txid}` (over 256 KiB: `record` and `old_record` null, `truncated: true` and `primary_key`). Delivery is at least once (dedupe on `id`): a non-2xx answer is retried with backoff, and the 10th failure makes the event dead (`dead_count`). Events queue while the function is disabled; a pending event is never dropped, and dead events past 10,000 per database or 7 days are removed and counted (`dropped_count`). The database's compute stays awake while it has an installed table trigger. A `storage` trigger starts `installed`. A new branch inherits the triggers.