Also known as Ephemeral Dust.
We try to ensure that dust is spent in blocks by requiring:
- ephemeral dust tx is 0-fee
- ephemeral dust tx only has one dust output
- If the ephemeral dust transaction has a child,
the dust is spent by by that child.
0-fee requirement means there is no incentive to mine
a transaction which doesn't have a child bringing its
own fees for the transaction package.
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