[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/kernel/mempool_entry.h

@@ -176,4 +176,6 @@ public:
     mutable Epoch::Marker m_epoch_marker; //!< epoch when last touched, useful for graph algorithms
 };
 
+using CTxMemPoolEntryRef = CTxMemPoolEntry::CTxMemPoolEntryRef;
+
 #endif // BITCOIN_KERNEL_MEMPOOL_ENTRY_H