Adding Dandelion++ support to public networks:

- New flag in NOTIFY_NEW_TRANSACTION to indicate stem mode
  - Stem loops detected in tx_pool.cpp
  - Embargo timeout for a blackhole attack during stem phase

tests/core_tests/tx_pool.h

@@ -127,3 +127,12 @@ struct txpool_double_spend_keyimage : txpool_double_spend_base
 
   bool generate(std::vector<test_event_entry>& events) const;
 };
+
+struct txpool_stem_loop : txpool_double_spend_base
+{
+  txpool_stem_loop()
+    : txpool_double_spend_base()
+  {}
+
+  bool generate(std::vector<test_event_entry>& events) const;
+};