[mempool] Only add a transaction to the unbroadcast set when it's added to the mempool

Currently, if BroadcastTransaction() is called to rebroadcast a
transaction (e.g. by ResendWalletTransactions()), then we add the
transaction to the unbroadcast set. That transaction has already been
broadcast in the past, so peers are unlikely to request it again,
meaning RemoveUnbroadcastTx() won't be called and it won't be removed
from m_unbroadcast_txids.

Net processing will therefore continue to attempt rebroadcast for the
transaction every 10-15 minutes. This will most likely continue until
the node connects to a new peer which hasn't yet seen the transaction
(or perhaps indefinitely).

Fix by only adding the transaction to the broadcast set when it's added
to the mempool.

Author: jnewbery

src/node/transaction.cpp

@@ -71,6 +71,12 @@ TransactionError BroadcastTransaction(NodeContext& node, const CTransactionRef t
 
         // Transaction was accepted to the mempool.
 
+        if (relay) {
+            // the mempool tracks locally submitted transactions to make a
+            // best-effort of initial broadcast
+            node.mempool->AddUnbroadcastTx(hashTx);
+        }
+
         if (wait_callback) {
             // For transactions broadcast from outside the wallet, make sure
             // that the wallet has been notified of the transaction before
@@ -96,9 +102,6 @@ TransactionError BroadcastTransaction(NodeContext& node, const CTransactionRef t
     }
 
     if (relay) {
-        // the mempool tracks locally submitted transactions to make a
-        // best-effort of initial broadcast
-        node.mempool->AddUnbroadcastTx(hashTx);
         node.peerman->RelayTransaction(hashTx, tx->GetWitnessHash());
     }