[refactor] add option to disable RBF

This is a mere refactor for now. We will use this to disable RBFing in
package validation.

src/validation.cpp

@@ -470,6 +470,8 @@ class MemPoolAccept
          */
         std::vector<COutPoint>& m_coins_to_uncache;
         const bool m_test_accept;
+        /** Disable BIP125 RBFing; disallow all conflicts with mempool transactions. */
+        const bool disallow_mempool_conflicts;
     };
 
     // Single transaction acceptance
@@ -631,7 +633,7 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
                         break;
                     }
                 }
-                if (fReplacementOptOut) {
+                if (fReplacementOptOut || args.disallow_mempool_conflicts) {
                     return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "txn-mempool-conflict");
                 }
 
@@ -1072,7 +1074,8 @@ static MempoolAcceptResult AcceptToMemoryPoolWithTime(const CChainParams& chainp
                                                       EXCLUSIVE_LOCKS_REQUIRED(cs_main)
 {
     std::vector<COutPoint> coins_to_uncache;
-    MemPoolAccept::ATMPArgs args { chainparams, nAcceptTime, bypass_limits, coins_to_uncache, test_accept };
+    MemPoolAccept::ATMPArgs args { chainparams, nAcceptTime, bypass_limits, coins_to_uncache,
+                                   test_accept, /* disallow_mempool_conflicts */ false };
 
     assert(std::addressof(::ChainstateActive()) == std::addressof(active_chainstate));
     const MempoolAcceptResult result = MemPoolAccept(pool, active_chainstate).AcceptSingleTransaction(tx, args);