Add MinPointsToBalance to nomination pools (#11377)

* add MinPointsToBalance to pools

* add min_points_to_balance to benchmark mock

* fmt

* comments

Co-authored-by: Oliver Tale-Yazdi <[email protected]>

* check min_points_to_balance.is_zero

* comment

* comment

* storage to constant

* fix

* comment

Co-authored-by: Oliver Tale-Yazdi <[email protected]>
Co-authored-by: parity-processbot <>