Change Dandelion++ fluff probability to 20%, and embargo timeout to 39s

A 20% fluff probability increases the precision of a spy connected to
every node by 10% on average, compared to a network using 0% fluff
probability. The current value (10% fluff) should increase precision by
~5% compared to baseline.

This decreases the expected stem length from 10 to 5. The embargo
timeout was therefore lowered to 39s; the fifth node in a stem is
expected to have a 90% chance of being the first to timeout, which is
the same probability we currently have with an expected stem length of
10 nodes.

src/cryptonote_core/tx_pool.cpp

@@ -75,11 +75,11 @@ namespace cryptonote
         not ideal since a blackhole is more likely to reveal earlier nodes in
         the chain.
 
-        This value was calculated with k=10, ep=0.10, and hop = 175 ms. A
+        This value was calculated with k=5, ep=0.10, and hop = 175 ms. A
         testrun from a recent Intel laptop took ~80ms to
         receive+parse+proces+send transaction. At least 50ms will be added to
         the latency if crossing an ocean. So 175ms is the fudge factor for
-        a single hop with 173s being the embargo timer. */
+        a single hop with 39s being the embargo timer. */
     constexpr const std::chrono::seconds dandelionpp_embargo_average{CRYPTONOTE_DANDELIONPP_EMBARGO_AVERAGE};
 
     //TODO: constants such as these should at least be in the header,