[refactor] rewrite vTxHashes as a vector of CTransactionRef

vTxHashes exposes a complex mapTx iterator type that its external users
don't need. Directly populate it with CTransactionRef instead.

src/txmempool.h

@@ -392,7 +392,7 @@ public:
     indexed_transaction_set mapTx GUARDED_BY(cs);
 
     using txiter = indexed_transaction_set::nth_index<0>::type::const_iterator;
-    std::vector<std::pair<uint256, txiter>> vTxHashes GUARDED_BY(cs); //!< All tx witness hashes/entries in mapTx, in random order
+    std::vector<CTransactionRef> vTxHashes GUARDED_BY(cs); //!< All transactions in mapTx, in random order
 
     typedef std::set<txiter, CompareIteratorByHash> setEntries;