wallet2: bias fake outs more towards recent outputs

Two recent papers quantified the real usage bias for the
real output in a ring being the true one, and shows that
the current biasing is much too weak.

While we wait for a better solution, we increase the ratio
of recent-to-total fake outputs, as well as decrease the
time window for recent outputs, so that half the fake outs
are selected within the last 1.8 day. Value plucked from
figure 10, page 11 of An Empirical Analysis of Linkability
in the Monero Blockchain, 2017, Miller et al.

This is also arbitrary, of course, but serves as a stopgap
till a better selection algorithm is chosen.

src/wallet/wallet2.cpp

@@ -82,8 +82,8 @@ using namespace cryptonote;
 #define UNSIGNED_TX_PREFIX "Monero unsigned tx set\003"
 #define SIGNED_TX_PREFIX "Monero signed tx set\003"
 
-#define RECENT_OUTPUT_RATIO (0.25) // 25% of outputs are from the recent zone
-#define RECENT_OUTPUT_ZONE (5 * 86400) // last 5 days are the recent zone
+#define RECENT_OUTPUT_RATIO (0.5) // 50% of outputs are from the recent zone
+#define RECENT_OUTPUT_ZONE ((time_t)(1.8 * 86400)) // last 1.8 day makes up the recent zone (taken from monerolink.pdf, Miller et al)
 
 #define FEE_ESTIMATE_GRACE_BLOCKS 10 // estimate fee valid for that many blocks