wallet/author: use vsize when estimating fees
wallet/author: use vsize when estimating fees

This commit makes use of the recently added EstimateVirtualSize
method to estimated the size of a transaction when calculating
fees. This makes fee estimation more accurate when we are spending
segwit outputs, as before we wouldn't account for the witness
descount, resulting in overshooting fee estimates.

Roasbeef/btcwallet,Roasbeef/btcwallet