rpcserver: enforce fee floor for fee rates input by the user
rpcserver: enforce fee floor for fee rates input by the user

In a previous commit, we modified our fee rates to be sat/kw internally
as it was possible that the estimated fee rate was not enough to
properly broadcast the transaction. To remedy this, we decided to add a
fee floor, but this would only be enforced when querying the fee
estimator. In this commit, we attempt to do the same thing, but for fee
rates input manually by the user.

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