lnwallet: in BtcdFeeEstimator is sat/byte is too low, fallback to default rate
lnwallet: in BtcdFeeEstimator is sat/byte is too low, fallback to default rate

In this commit, we fix an existing bug within the EstimateFeePerWeight
method for the BtcdFeeEstimator. If the sat/byte value returned was too
low, then it was possible for us to end up with a zero valued
sat/weight. We correct this issue by detecting, and falling back to the
default fee rate if so.

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