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/reference/connector-contract.md

@@ -201,6 +201,45 @@ a remote panel wants a frame every second or two; higher rates than that are
 for the local network, and full-rate video still belongs on the camera's own
 stream.
 
+## Keeping state across restarts
+
+A connection, a session or a device handle lives in the connector's own
+attributes and goes with the process. The few things that must not, such as
+the certificate a device presented on first contact or a token it paired with,
+go in `self.store`:
+
+```python
+pem = await asyncio.to_thread(self.store.get, f"cert:{self.config.host}")
+await asyncio.to_thread(self.store.set, f"cert:{self.config.host}", pem)
+await asyncio.to_thread(self.store.delete, f"cert:{self.config.host}")
+```
+
+- Keys and values are strings; `get` returns `None` for a key that is not
+  there, and `delete` returns whether there was one.
+- The store belongs to the connector's **top-level package**, not to a node:
+  every node of the package sees the same keys, so two nodes on one bridge
+  share what was stored for it. Put the device in the key.
+- It lives in the engine's database, so it survives a restart and needs
+  nothing `fluksio serve` does not have. It is available before `start`.
+- Limits: a key is 1 to 200 characters without `/` or whitespace, a value at
+  most 64 kB, and a package at most 256 keys. Past any of them, `set` raises
+  `ValueError`. Bytes worth keeping belong in `save_artifact`.
+- Every call is a database round trip, so on the event loop it goes through
+  `asyncio.to_thread`.
+
+A superuser can see the keys (never the values) and clear them, all of them or
+one at a time, under *Stored state* in a connector node's settings, or through
+the API, which names the connector by any node type it installs:
+
+```sh
+curl -s $FLUKSIO/connectors/hue/state -H "Authorization: Bearer $TOKEN"
+curl -X DELETE $FLUKSIO/connectors/hue/state/cert:10.0.0.2 -H "Authorization: Bearer $TOKEN"
+curl -X DELETE $FLUKSIO/connectors/hue/state -H "Authorization: Bearer $TOKEN"
+```
+
+So a connector has to cope with a key disappearing: clearing is how somebody
+tells it to start over.
+
 ## Lifecycle
 
 ```python
@@ -239,7 +278,8 @@ load is reported separately and never runs.
 
 - **Hold state that belongs in the flow.** Logic nodes are stateless and run in
   parallel; a connector may hold a connection, a session, or a device handle,
-  but a value another node needs belongs in a message.
+  and keep what has to outlive a restart in `self.store`, but a value another
+  node needs belongs in a message.
 - **Block the event loop.** `poll`, `start` and `stop` are async and run on the
   loop. Use `asyncio.to_thread` for blocking I/O.
 - **Read credentials from the environment.** They come through parameters, so