Introduce mempoolfullrbf node setting.

This new node policy setting enables to accept replaced-by-fee
transaction without inspection of the replaceability signaling
as described in BIP125 "explicit signaling".

If turns on, the node mempool accepts transaction replacement
as described in `policy/mempool-replacements.md`.

The default setting value is `false`, implying opt-in RBF
is enforced.

src/validation.cpp

@@ -740,7 +740,10 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
                 // Applications relying on first-seen mempool behavior should
                 // check all unconfirmed ancestors; otherwise an opt-in ancestor
                 // might be replaced, causing removal of this descendant.
-                if (!SignalsOptInRBF(*ptxConflicting)) {
+                //
+                // If replaceability signaling is ignored due to node setting,
+                // replacement is always allowed.
+                if (!m_pool.m_full_rbf && !SignalsOptInRBF(*ptxConflicting)) {
                     return state.Invalid(TxValidationResult::TX_MEMPOOL_POLICY, "txn-mempool-conflict");
                 }