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_basic/cryptonote_basic_impl.h

@@ -69,6 +69,7 @@ namespace cryptonote {
   /************************************************************************/
   /* Cryptonote helper functions                                          */
   /************************************************************************/
+  size_t get_min_block_size(uint8_t version);
   size_t get_max_block_size();
   size_t get_max_tx_size();
   bool get_block_reward(size_t median_size, size_t current_block_size, uint64_t already_generated_coins, uint64_t &reward, uint8_t version);