graph/db: fix race in DisconnectBlockAtHeight cache access

The DisconnectBlockAtHeight method was modifying the rejectCache and
chanCache without holding the cacheMu lock. This caused races with
other operations that properly held the lock, such as AddChannelEdge
which modifies the caches in its OnCommit callback while the batch
scheduler holds cacheMu.

Fix by acquiring cacheMu before removing channels from the caches.

graph/db/sql_store.go

@@ -2911,10 +2911,12 @@ func (s *SQLStore) DisconnectBlockAtHeight(height uint32) (
 			"height: %w", err)
 	}
 
+	s.cacheMu.Lock()
 	for _, channel := range removedChans {
 		s.rejectCache.remove(channel.ChannelID)
 		s.chanCache.remove(channel.ChannelID)
 	}
+	s.cacheMu.Unlock()
 
 	return removedChans, nil
 }