Add intervening v5 fork for increased min block size

Minimum mixin 4 and enforced ringct is moved from v5 to v6.
v5 is now used for an increased minimum block size (from 60000
to 300000) to cater for larger typical/minimum transaction size.

The fee algorithm is also changed to decrease the base per kB
fee, and add a cheap tier for those transactions which we do
not care if they get delayed (or even included in a block).

src/cryptonote_core/blockchain.h

@@ -520,10 +520,11 @@ namespace cryptonote
      *
      * @param block_reward the current block reward
      * @param median_block_size the median blob's size in the past window
+     * @param version hard fork version for rules and constants to use
      *
      * @return the per kB fee
      */
-    static uint64_t get_dynamic_per_kb_fee(uint64_t block_reward, size_t median_block_size);
+    static uint64_t get_dynamic_per_kb_fee(uint64_t block_reward, size_t median_block_size, uint8_t version);
 
     /**
      * @brief get dynamic per kB fee estimate for the next few blocks