[net] Change AdvertiseLocal to GetLocalAddrForPeer

Gossiping addresses to peers is the responsibility of net processing.
Change AdvertiseLocal() in net to just return an (optional) address
for net processing to advertise. Update function name to reflect
new responsibility.

src/net.h

@@ -197,7 +197,8 @@ enum
 };
 
 bool IsPeerAddrLocalGood(CNode *pnode);
-void AdvertiseLocal(CNode *pnode);
+/** Returns a local address that we should advertise to this peer */
+Optional<CAddress> GetLocalAddrForPeer(CNode *pnode);
 
 /**
  * Mark a network as reachable or unreachable (no automatic connects to it)