core: dynamic fee algorithm from ArticMine

The fee will vary based on the base reward and the current
block size limit:

fee = (R/R0) * (M0/M) * F0

R: base reward
R0: reference base reward (10 monero)
M: block size limit
M0: minimum block size limit (60000)
F0: 0.002 monero

Starts applying at v4

src/cryptonote_core/tx_pool.cpp

@@ -133,12 +133,8 @@ namespace cryptonote
       fee = tx.rct_signatures.txnFee;
     }
 
-    uint64_t needed_fee = blob_size / 1024;
-    needed_fee += (blob_size % 1024) ? 1 : 0;
-    needed_fee *= FEE_PER_KB;
-    if (!kept_by_block && fee < needed_fee)
+    if (!kept_by_block && !m_blockchain.check_fee(blob_size, fee))
     {
-      LOG_PRINT_L1("transaction fee is not enough: " << print_money(fee) << ", minimum fee: " << print_money(needed_fee));
       tvc.m_verifivation_failed = true;
       tvc.m_fee_too_low = true;
       return false;