get_block_template_backlog: better sorting logic

std::sort is unstable, so it can return random sets of transactions when mempool has many transactions with the same fee/byte. It can result in p2pool mining empty blocks sometimes because it doesn't pick up "new" transactions immediately.

src/cryptonote_core/tx_pool.cpp

@@ -1074,7 +1074,7 @@ namespace cryptonote
 
     // If the total weight is too high, choose the best paying transactions
     if (total_weight > max_backlog_weight)
-      std::sort(tmp.begin(), tmp.end(), [](const auto& a, const auto& b){ return a.fee * b.weight > b.fee * a.weight; });
+      std::stable_sort(tmp.begin(), tmp.end(), [](const auto& a, const auto& b){ return a.fee * b.weight > b.fee * a.weight; });
 
     backlog.clear();
     uint64_t w = 0;