[btcsuite/btcd] 489ba8: Update minimum fee calculations to match Core.

[Ben Echols]

  Branch: refs/heads/master
  Home:   https://github.com/btcsuite/btcd
  Commit: 489ba8d31d2c54ec2e1043396bc0e844c6253844
      
https://github.com/btcsuite/btcd/commit/489ba8d31d2c54ec2e1043396bc0e844c6253844
  Author: Ben Echols <lologari...@gmail.com>
  Date:   2015-10-30 (Fri, 30 Oct 2015)

  Changed paths:
    M policy.go
    M policy_test.go

  Log Message:
  -----------
  Update minimum fee calculations to match Core.

Calculate rate*fee first before dividing by 1000.
Add more unit tests around the fee calculations.