Fix issue with conflicted mempool tx in listsinceblock (Adam Jonas)
Pull request description:
Closes #8752 by bringing back abandoned #10470.
This now checks that returned transactions are not conflicting with any transactions that are filtered out by the given blockhash and add a functional test to prevent this in the future.
For more context, #8757 was closed in favor of #10470.
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Backport of Core PR17258