routing: fix bug in channel capacity display by ensuring properly typed arithmetic
routing: fix bug in channel capacity display by ensuring properly typed arithmetic

This commit fixes a slight bug in the storage of the capacity of a
channel. Previously, we were subtracting a the hard coded fee amount
without first casting the integer to a btcutil.Amount which results in
a display/rounding error when the amount is converted to BTC.

LightningNetwork/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,ddustin/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,LightningNetwork/lnd,ddustin/lnd