This commit adds `TxPublisher` which implements `Bumper` interface. This
is part one of the implementation that focuses on implementing the
`Broadcast` method which guarantees a tx can be published with
RBF-compliant. It does so by leveraging the `testmempoolaccept` API,
keep increasing the fee rate until an RBF-compliant tx is made and
broadcasts it.
This tx will then be monitored by the `TxPublisher` and in the following
commit, the monitoring process will be added.
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.
