policy: Allow dust in transactions, spent in-mempool

Also known as Ephemeral Dust.

We try to ensure that dust is spent in blocks by requiring:
  - ephemeral dust tx is 0-fee
  - ephemeral dust tx only has one dust output
  - If the ephemeral dust transaction has a child,
    the dust is spent by by that child.

0-fee requirement means there is no incentive to mine
a transaction which doesn't have a child bringing its
own fees for the transaction package.

src/policy/policy.cpp

@@ -129,6 +129,7 @@ bool IsStandardTx(const CTransaction& tx, const std::optional<unsigned>& max_dat
     }
 
     unsigned int nDataOut = 0;
+    unsigned int num_dust_outputs{0};
     TxoutType whichType;
     for (const CTxOut& txout : tx.vout) {
         if (!::IsStandard(txout.scriptPubKey, max_datacarrier_bytes, whichType)) {
@@ -142,11 +143,16 @@ bool IsStandardTx(const CTransaction& tx, const std::optional<unsigned>& max_dat
             reason = "bare-multisig";
             return false;
         } else if (IsDust(txout, dust_relay_fee)) {
-            reason = "dust";
-            return false;
+            num_dust_outputs++;
         }
     }
 
+    // Only MAX_DUST_OUTPUTS_PER_TX dust is permitted(on otherwise valid ephemeral dust)
+    if (num_dust_outputs > MAX_DUST_OUTPUTS_PER_TX) {
+        reason = "dust";
+        return false;
+    }
+
     // only one OP_RETURN txout is permitted
     if (nDataOut > 1) {
         reason = "multi-op-return";