This commit adds `BudgetAggregator` as a new implementation of
`UtxoAggregator`. This aggregator will group inputs by their deadline
heights and create input sets that can be used directly by the fee
bumper for fee calculations.
This commit makes sure an input is only added to the cluster when it has
successfully estimated its fee rate. Previously, when an error is
returned from `feeRateForPreference`, we'd still add this input to the
cluster, resulting a **lower** fee rates being used because when
averaging the fee rates, we'd think this input has zero fee rate
specified.
An unit test is patched to make the method `clusterByLockTime` more
robust.
