[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/test/txvalidationcache_tests.cpp

@@ -28,9 +28,9 @@ BOOST_FIXTURE_TEST_CASE(tx_mempool_block_doublespend, TestChain100Setup)
     const auto ToMemPool = [this](const CMutableTransaction& tx) {
         LOCK(cs_main);
 
-        TxValidationState state;
-        return AcceptToMemoryPool(*m_node.mempool, state, MakeTransactionRef(tx),
-            nullptr /* plTxnReplaced */, true /* bypass_limits */);
+        const MempoolAcceptResult result = AcceptToMemoryPool(*m_node.mempool, MakeTransactionRef(tx),
+            true /* bypass_limits */);
+        return result.m_result_type == MempoolAcceptResult::ResultType::VALID;
     };
 
     // Create a double-spend of mature coinbase txn: