[mempool] Track "unbroadcast" transactions

- Mempool tracks locally submitted transactions (wallet or rpc)
- Transactions are removed from set when the node receives a GETDATA request
  from a peer, or if the transaction is removed from the mempool.

src/node/transaction.cpp

@@ -78,6 +78,10 @@ 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);
+
         RelayTransaction(hashTx, *node.connman);
     }