core: add a few more block rate window sizes

The 10 minute one will never trigger for 0 blocks, as it's still
fairly likely to happen even without the actual hash rate changing
much, so we add a 20 minute window, where it will (for 0 blocks)
and a one hour window.

src/cryptonote_core/cryptonote_core.cpp

@@ -1788,7 +1788,7 @@ namespace cryptonote
     const time_t now = time(NULL);
     const std::vector<time_t> timestamps = m_blockchain_storage.get_last_block_timestamps(60);
 
-    static const unsigned int seconds[] = { 5400, 1800, 600 };
+    static const unsigned int seconds[] = { 5400, 3600, 1800, 1200, 600 };
     for (size_t n = 0; n < sizeof(seconds)/sizeof(seconds[0]); ++n)
     {
       unsigned int b = 0;