Give connectors a store that survives restarts, and a way to clear it

`ConnectorNode.store` keeps short strings per connector package in the
engine's database (`connector_state`, migration f8c3e5a91d27), so the nodes of
one package share them and `fluksio serve` needs nothing new: get, set and
delete, at most 64 kB a value and 256 keys a package. A superuser lists the
keys (never the values) and clears them through `/connectors/{node_type}/state`,
or under Stored state in a connector node's panel. Documented in the connector
contract.

docs/code/api.md

@@ -204,7 +204,10 @@ curl -X POST $FLUKSIO/alerts/test/my-phone -H "Authorization: Bearer $TOKEN"
 ```
 
 Secrets are write-only over the API: you can list names and set values, never
-read one back. Secrets and `POST /modules/apply` are a superuser's.
+read one back. Secrets and `POST /modules/apply` are a superuser's, and so is
+what a connector keeps across restarts: `GET` or `DELETE
+/connectors/{node_type}/state`, or `DELETE /connectors/{node_type}/state/{key}`
+for one key (see [the connector contract](../reference/connector-contract.md#keeping-state-across-restarts)).
 
 ## Observability