message: Add "bip125-replaceable" flag to mempool RPCs
  This affects getrawmempool, getmempoolentry, getmempoolancestors and getmempooldescendants.
diff: #include <core_io.h>
  #include <policy/feerate.h>
  #include <policy/policy.h>
  +#include <policy/rbf.h>
  #include <primitives/transaction.h>
  #include <rpc/server.h>
  #include <streams.h>
  @@ -376,7 +377,8 @@ std::string EntryDescriptionString()
  " ... ]\n"
  " \"spentby\" : [ (array) unconfirmed transactions spending outputs from this transaction\n"
  " \"transactionid\", (string) child transaction id\n"
  - " ... ]\n";
  + " ... ]\n"
  + " \"bip125-replaceable\" : true|false, (boolean) Whether this transaction could be replaced due to BIP125 (replace-by-fee)\n";
  }
  void entryToJSON(UniValue &info, const CTxMemPoolEntry &e)
  @@ -419,6 +421,17 @@ void entryToJSON(UniValue &info, const CTxMemPoolEntry &e)
  }
  info.pushKV("spentby", spent);
  +
  + // Add opt-in RBF status
  + bool rbfStatus = false;
  + RBFTransactionState rbfState = IsRBFOptIn(tx, mempool);
  + if (rbfState == RBFTransactionState::UNKNOWN) {
  + throw JSONRPCError(RPC_MISC_ERROR, "Transaction is not in mempool");
  + } else if (rbfState == RBFTransactionState::REPLACEABLE_BIP125) {
  + rbfStatus = true;
  + }
  +
  + info.pushKV("bip125-replaceable", rbfStatus);
  }
  UniValue mempoolToJSON(bool fVerbose)
  