[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/test/net_tests.cpp

@@ -691,7 +691,7 @@ BOOST_AUTO_TEST_CASE(ipv4_peer_with_ipv6_addrMe_test)
     pnode->SetAddrLocal(addrLocal);
 
     // before patch, this causes undefined behavior detectable with clang's -fsanitize=memory
-    AdvertiseLocal(&*pnode);
+    GetLocalAddrForPeer(&*pnode);
 
     // suppress no-checks-run warning; if this test fails, it's by triggering a sanitizer
     BOOST_CHECK(1);