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/connectors.md

@@ -143,3 +143,4 @@ raised exception shows there with its traceback and marks the node down.
 - `poll()` returns `None` rather than raising when there is simply nothing new.
 - `write()` returns quickly; the thread it runs on is the scheduler's.
 - Health reported when the connection comes and goes.
+- Anything that has to outlive a restart in `self.store`, keyed by device.