[0.25 XMR] Take fee of unconfirmed parent into consideration when deciding fee

[binarybaron]

Withdrawals from exchanges often have a very low fee. If we want to get our Bitcoin lock transaction get confirmed faster, we need to set the fee such that the cumulative fee is high enough for miners to confirm our transaction.

[binarybaron]

This issue has been assigned a bounty (💰)

Current bounty:

• 0.25 XMR contributed by @binarybaron and @Einliterflasche
• 0 XMR contributed by the eigenwallet community

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