[BUG] Calculate the avg fee rate from block

Heimdallrequested to merge
1217-issue into develop

Related to #1217 (closed)

The DOGE chain client was implemented based on https://github.com/dogecoin/dogecoin/blob/master/doc/fee-recommendation.md , which recommended a fix fee rate 0.01 DOGE / Kb , however it turns out there are a lot transactions pay far more than that, so update DOGE chain client to calculated the avg fee rate , and let the network to pay higher fee to get the tx mined quicker.

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