[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.cpp

@@ -862,6 +862,13 @@ std::vector<TxMempoolInfo> CTxMemPool::infoAll() const
     return ret;
 }
 
+const CTxMemPoolEntry* CTxMemPool::GetEntry(const Txid& txid) const
+{
+    AssertLockHeld(cs);
+    const auto i = mapTx.find(txid);
+    return i == mapTx.end() ? nullptr : &(*i);
+}
+
 CTransactionRef CTxMemPool::get(const uint256& hash) const
 {
     LOCK(cs);