Ensure we set funding amounts on old channels still on the graph

In c02f2ec we added
`funding_amount_sats` to `channel_announcements`, making it
`NOT NULL` for new DBs, but `NULL` for old ones. When querying,
however, we (indirectly) `unwrap`ed the values.

Here, we try to backfill by looking at the local network graph on
startup and updating the existing entries, rather than ignoring
conflicting writes.

Author: TheBlueMatt

src/persistence.rs

@@ -201,7 +201,7 @@ impl<L: Deref + Clone + Send + Sync + 'static> GossipPersister<L> where L::Targe
 								short_channel_id, \
 								funding_amount_sats, \
 								announcement_signed \
-							) VALUES ($1, $2, $3) ON CONFLICT (short_channel_id) DO NOTHING", &[
+							) VALUES ($1, $2, $3) ON CONFLICT (short_channel_id) DO UPDATE", &[
 									&scid,
 									&(funding_value as i64),
 									&announcement_signed