Use larger nPruneAfterSizeIn parameter for mapAlreadyAskedFor (#2882)

unordered_limitedmap was meant to be used with much larger
nPruneAfterSizeIn values to maintain good performance. The way it is not
will result in pruning too often on high load.

src/net.cpp

@@ -85,7 +85,7 @@ std::map<CNetAddr, LocalServiceInfo> mapLocalHost;
 static bool vfLimited[NET_MAX] = {};
 std::string strSubVersion;
 
-unordered_limitedmap<uint256, int64_t, StaticSaltedHasher> mapAlreadyAskedFor(MAX_INV_SZ);
+unordered_limitedmap<uint256, int64_t, StaticSaltedHasher> mapAlreadyAskedFor(MAX_INV_SZ, MAX_INV_SZ * 2);
 
 // Signals for message handling
 static CNodeSignals g_signals;