[refactor] return MempoolAcceptResult

This creates a cleaner interface with ATMP, allows us to make results const,
and makes accessing values that don't make sense (e.g. fee when tx is
invalid) an error.

src/bench/block_assemble.cpp

@@ -48,9 +48,8 @@ static void AssembleBlock(benchmark::Bench& bench)
         LOCK(::cs_main); // Required for ::AcceptToMemoryPool.
 
         for (const auto& txr : txs) {
-            TxValidationState state;
-            bool ret{::AcceptToMemoryPool(*test_setup.m_node.mempool, state, txr, nullptr /* plTxnReplaced */, false /* bypass_limits */)};
-            assert(ret);
+            const MempoolAcceptResult res = ::AcceptToMemoryPool(*test_setup.m_node.mempool, txr, false /* bypass_limits */);
+            assert(res.m_result_type == MempoolAcceptResult::ResultType::VALID);
         }
     }