Allows certain sweep inputs to be kept in separate transactions at all
times. This is a preparation for anchor outputs. Before the commitment
tx confirms, there are three potential anchors that can be cpfp'ed. We
want to cpfp them all, but if done in the same transaction, the
transaction would guaranteed to be invalid. Exponential backoff would
eventually get the txes published, but having exclusive groups makes the
process faster.
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