* [NOD-1006] Make CompactToBig take an out param so that we can reuse the same big.Int in averageTarget.
* [NOD-1006] Fix merge errors.
* [NOD-1006] Use CompactToBigWithDestination only in averageTarget.
* [NOD-1006] Fix refactor errors.
* [NOD-1006] Fix refactor errors.
* [NOD-1006] Optimize averageTarget with a big.Int pool.
* [NOD-1006] Defer releasing bigInts.
* [NOD-1006] Use a pool for requiredDifficulty as well.
* [NOD-1006] Move the big int pool to utils.
* [NOD-1006] Remove unnecessary line.