Persist `NetworkGraph` in BP upon RGS updates

[tnull]

Currently, we persist the network graph after a minute, then hourly, and finally on shutdown.

As the hourly interval is very conservative and mostly makes sense on a 24/7 server and the initial delay of 1 minute could be too long or too short, we should switch to a polling-based logic in case we get our graph data via RapidGossipSync.

[TheBlueMatt]

Oops I think we had a minor regression here, too, we used to always poll after 60s but now with #2081 we check after 60s and then poll every 3600s. Of course with RGS you almost always complete within 60s and the app is usually closed before 60s, so its not a big regression - we were screwed already, but worth noting.

[TheBlueMatt]

Slipping to 116, this is pretty easy to do but not super critical. While we're at it we should also persist the scorer any time its been updated based on the event handler, rather than (or in addition to) on a timer.