[refactor] Add helper for retrieving mempool entry

In places where the iterator is only needed for accessing the actual
entry, it should not be required to first retrieve the iterator.

src/txmempool.h

@@ -684,6 +684,8 @@ public:
         return (mapTx.count(gtxid.GetHash()) != 0);
     }
 
+    const CTxMemPoolEntry* GetEntry(const Txid& txid) const LIFETIMEBOUND EXCLUSIVE_LOCKS_REQUIRED(cs);
+
     CTransactionRef get(const uint256& hash) const;
     txiter get_iter_from_wtxid(const uint256& wtxid) const EXCLUSIVE_LOCKS_REQUIRED(cs)
     {