Support greedy addressees

Setting the 'greedy' flag on an addressee means it will consume any
change instead of it going to a change output.

src/ga_tx.cpp

@@ -572,14 +572,20 @@ namespace sdk {
 
                 // Add all outputs and compute the total amount of satoshi to be sent
                 amount required_total{ 0 };
+                uint32_t greedy_index = NO_CHANGE_INDEX;
 
                 if (num_addressees) {
+                    int addressee_index = 0;
                     for (auto& addressee : *addressees_p) {
                         const auto addressee_asset_id = asset_id_from_json(net_params, addressee);
                         if (addressee_asset_id == asset_id) {
                             required_total += add_tx_addressee(session, net_params, result, tx, addressee);
                             reordered_addressees.push_back(addressee);
+                            if (addressee.value("is_greedy", false)) {
+                                greedy_index = addressee_index;
+                            }
                         }
+                        ++addressee_index;
                     }
                 }
 
@@ -745,6 +751,14 @@ namespace sdk {
 
                     change = total - required_with_fee;
 
+                    if (change != 0 && greedy_index != NO_CHANGE_INDEX) {
+                        // If a 'greedy' addressee exists send change there
+                        set_tx_output_value(net_params, tx, greedy_index, asset_id, change.value());
+                        addressees_p->at(greedy_index)["satoshi"] = change.value();
+                        required_total += change;
+                        continue;
+                    }
+
                     if ((!have_change_output && change < dust_threshold)
                         || (have_change_output && change >= dust_threshold)) {
                         // We don't have a change output, and have only dust left over, or

src/transaction_utils.cpp

@@ -424,7 +424,10 @@ namespace sdk {
 
         // Transactions with outputs below the dust threshold (except OP_RETURN)
         // are not relayed by network nodes
-        if (!result.value("send_all", false) && satoshi.value() < session.get_dust_threshold()) {
+        // This check only applies when the amount has been set explicitly, so send all
+        // and greedy outputs are ignored as the amount is calculated later
+        if (!result.value("send_all", false) && !addressee.value("is_greedy", false)
+            && satoshi.value() < session.get_dust_threshold()) {
             set_tx_error(result, res::id_invalid_amount);
         }