blockchain: lower the relay fee by 2%

This ensures we accept txes with a fee that's slightly too small,
to accomodate blockchain median jitter

src/cryptonote_core/blockchain.cpp

@@ -2860,7 +2860,7 @@ bool Blockchain::check_fee(size_t blob_size, uint64_t fee) const
   needed_fee += (blob_size % 1024) ? 1 : 0;
   needed_fee *= fee_per_kb;
 
-  if (fee < needed_fee)
+  if (fee < needed_fee * 0.98) // keep a little buffer on acceptance
   {
     MERROR_VER("transaction fee is not enough: " << print_money(fee) << ", minimum fee: " << print_money(needed_fee));
     return false;