Update min relay fee calculation to match btcd calc
Update min relay fee calculation to match btcd calc

Plucked from this btcd commit:
https://github.com/btcsuite/btcd/commit/489ba8d31d2c54ec2e1043396bc0e844c6253844

Would have been cherry-picked but changes with policy.go prevent that
for now

jrick/btcd,jcmincke/dcrd,jrick/dcrd,jcmincke/dcrd,jrick/btcd,jcmincke/dcrd,jrick/btcd,decred/dcrd,decred/dcrd,jrick/dcrd