watchtower/wtpolicy: properly set DefaultSweepFeeRate and MinSweepFeeRate
watchtower/wtpolicy: properly set DefaultSweepFeeRate and MinSweepFeeRate

These values were intended to be 10 sat/vbyte and 4 sat/vbyte
respectively, but the conversion was done incorrectly.

aakselrod/lnd,aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd