adds fallback logic if retransmit multicast fails (#17714)

In retransmit-stage, based on the packet.meta.seed and resulting
children/neighbors, each packet is sent to a different set of peers:
https://github.com/solana-labs/solana/blob/708bbcb00/core/src/retransmit_stage.rs#L421-L457

However, current code errors out as soon as a multicast call fails,
which will skip all the remaining packets:
https://github.com/solana-labs/solana/blob/708bbcb00/core/src/retransmit_stage.rs#L467-L470

This can exacerbate packets loss in turbine.

This commit:
  * keeps iterating over retransmit packets for loop even if some
    intermediate sends fail.
  * adds a fallback to UdpSocket::send_to if multicast fails.

Recent discord chat:
https://discord.com/channels/428295358100013066/689412830075551748/849530845052403733

gossip/tests/gossip.rs

@@ -209,7 +209,7 @@ pub fn cluster_info_retransmit() {
     p.meta.size = 10;
     let peers = c1.tvu_peers();
     let retransmit_peers: Vec<_> = peers.iter().collect();
-    ClusterInfo::retransmit_to(&retransmit_peers, &p, &tn1, false).unwrap();
+    ClusterInfo::retransmit_to(&retransmit_peers, &p, &tn1, false);
     let res: Vec<_> = [tn1, tn2, tn3]
         .into_par_iter()
         .map(|s| {