Introduce a minimum fee per byte

CodeChain invalidates transactions whose fee is less than the minimum fee. The fee is charged for the network usage. We should set the minimum fee according to the transaction's CPU cost and Disk cost.

CodeChain requires a fixed fee for a Transfer transaction. A transfer transaction has multiple inputs and outputs. The transaction issuer should pay more fees if the size of the transaction is bigger.

I have one concern. There are two ways to apply the minimum fee: one is the consensus' minimum fee. CodeChain nodes will drop the transactions that have less than the consensus' minimum fee. The other one is each node's minimum fee. Each node can reject to propagate or to mine transactions that have small fees.

IMO, we should implement fee-per-byte minimum fees in both places.