[refactor] Add helper for iterating through mempool entries

Instead of reaching into the mapTx data structure, use a helper method
that provides the required vector of CTxMemPoolEntry pointers.

src/node/interfaces.cpp

@@ -806,7 +806,7 @@ public:
     {
         if (!m_node.mempool) return;
         LOCK2(::cs_main, m_node.mempool->cs);
-        for (const CTxMemPoolEntry& entry : m_node.mempool->mapTx) {
+        for (const CTxMemPoolEntry& entry : m_node.mempool->entryAll()) {
             notifications.transactionAddedToMempool(entry.GetSharedTx());
         }
     }