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

src/cryptonote_protocol/cryptonote_protocol_defs.h

@@ -197,10 +197,12 @@ namespace cryptonote
     {
       std::vector<blobdata>   txs;
       std::string _; // padding
+      bool dandelionpp_fluff; //zero initialization defaults to stem mode
 
       BEGIN_KV_SERIALIZE_MAP()
         KV_SERIALIZE(txs)
         KV_SERIALIZE(_)
+        KV_SERIALIZE_OPT(dandelionpp_fluff, true) // backwards compatible mode is fluff
       END_KV_SERIALIZE_MAP()
     };
     typedef epee::misc_utils::struct_init<request_t> request;