Add src/node/* code to node:: namespace

src/node/transaction.cpp

@@ -16,6 +16,7 @@
 
 #include <future>
 
+namespace node {
 static TransactionError HandleATMPError(const TxValidationState& state, std::string& err_string_out)
 {
     err_string_out = state.ToString();
@@ -153,3 +154,4 @@ CTransactionRef GetTransaction(const CBlockIndex* const block_index, const CTxMe
     }
     return nullptr;
 }
+} // namespace node