peer: don't reply if peer's request `initial_graph_sync`
peer: don't reply if peer's request `initial_graph_sync`

We stopped requesting this from other nodes as it is very expensive as
the graph continues to grow. In this commit we will also stop
responding, as nodes are recommended to begin using the `gossip_queries`
and upcoming `extended_gossip_queries` to reconcile missing graph data.

LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd