fix(core-transaction-pool): use sorted array (instead of tree) for storing transactions by fee and nonce

[air1one]

Summary

Use sorted array instead of tree for storing transaction by fee and nonce, because tree has some issues like with recursion and max call stack. Sorted array is simpler and does the job for a lower max size of the transaction pool (now reduced to 15k transactions by default instead of 100k).

(tree memory structure is still available in core-utils and might be improved and used in the future : needs re-balancing while inserting to be usable)

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged