core/txpool/legacypool: move queue out of main txpool (#32270)

This PR move the queue out of the main transaction pool.
For now there should be no functional changes.
I see this as a first step to refactor the legacypool and make the queue
a fully separate concept from the main pending pool.

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Signed-off-by: Csaba Kiraly <[email protected]>
Co-authored-by: Csaba Kiraly <[email protected]>

Author: MariusVanDerWijden