New fee rate: 0 on testnet

[reqlez]

For whatever reason, on testnet, the "feerate" is getting set to 0.

I was using this with bitcoinlib and took a bit to figure out why exactly my transactions were being rejected due to low fees.

bcoin_tn | [debug] (fees) Recalculating dynamic cutoffs.
bcoin_tn | [debug] (fees) Done updating estimates for 2 confirmed entries. New mempool map size 0.
bcoin_tn | [debug] (fees) New fee rate: 0.