Assign background transaction futures to the tracking array so they can be cancelled

[siladu]

This could improve performance in the cases where sequential execution beats parallel, particularly when dealing with a large transaction such as a XEN contract: if sequential processing catches up, cancel the parallel tx future and process the tx once sequentially instead of leaving the parallel tx future to complete in the background.

Testing

Test was run over 2.5 days, ~18K blocks.

Full analysis report: https://hackmd.io/@siladu/HypqnrhCxl

Analysis shows no regression and perhaps some limited improvements to specific blocks, though comparing to all our running nodes' block times, I think this is likely noise.
One outlier result for block 23647133 skews the report quite a lot.

Files used in analysis

Besu Logs

• control-besu.log
• test-cancellation-besu.log

Analysis tool (usage in the hackmd report)

• compare_besu_exec.py

XEN contract transaction lists

• export-0xc3c7b049678d84081dfd0ba21a6c7fdcc31c226f_XEN_Minter.csv
• export-0x0de8bf93da2f7eecb3d9169422413a9bef4ef628_CoinTool_XEN_Batch_Minter.csv
• 0x2f848984984d6c3c036174ce627703edaf780479_MCT-XEN_Batch_Minter_export-transaction-list-1761523431099.csv
• 0x0000000000771a79d0fc7f3b7fe270eb4498f20b_MCT_MXENFT_Token_export-transaction-list.csv