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/chaingen.h

@@ -116,7 +116,8 @@ struct event_visitor_settings
   {
     set_txs_keeped_by_block = 1 << 0,
     set_txs_do_not_relay = 1 << 1,
-    set_local_relay = 1 << 2
+    set_local_relay = 1 << 2,
+    set_txs_stem = 1 << 3
   };
 
   event_visitor_settings(int a_mask = 0)
@@ -548,6 +549,10 @@ public:
     {
       m_tx_relay = cryptonote::relay_method::none;
     }
+    else if (settings.mask & event_visitor_settings::set_txs_stem)
+    {
+      m_tx_relay = cryptonote::relay_method::stem;
+    }
     else
     {
       m_tx_relay = cryptonote::relay_method::fluff;