Optimize Network Graph Loading

In `barq-plugin`, we currently load the entire network graph (including the gossip map) each time the `pay` command is called. This approach is redundant and can be inefficient, especially since the gossip map can be quite large (potentially in GBs).

To improve efficiency, we should:
- Store the network graph along with hints from the gossip map in the plugin state.
- Maybe implement a cron job or similar mechanism to update the network graph periodically.

Start by tracing the barq-plugin pay command and how plugin state currently loads the network graph and gossip map. Determine how graph hints can be retained and how periodic updates would fit the plugin; done means pay no longer reloads the entire graph on every call while the stored data can be refreshed.

Written by the indexing model from the issue text.