New BSIP: per-position force-settlement volume (percentage) limit

[BSIP38](https://github.com/bitshares/bsips/blob/master/bsip-0038.md) introduced the `target collateral ratio` (abbr. `TCR`) option so that debt position owners can decide how much collateral to sell when being margin called. However, the `TCR` option doesn't affect force-settlements. When being force-settled, the debt position with the least collateral ratio (abbr. `CR`) will buy as much debt asset as possible (capped by its total debt) to fill the matched force-settlement order.

Some debt position holders may also like to sell as little collateral as possible when being force-settled. Since a debt position's `CR` would raise after sold some collateral, it's possible that the position's new `CR` would be higher than other positions', thus it's fair for this position to not sell more collateral.

The new options could include
* a per-asset `default_per_position_force_settlement_percentage` option that can be defined by asset owners, floored by the `maximum_force_settlement_volume` percentage, and
* a per-position `force_settlement_percentage` option that can be defined by debt position owners, floored by the corresponding asset's `default_per_position_force_settlement_percentage` option.

When executing (filling) a force-settlement, let the debt position with the least `CR` buy up to `force_settlement_percentage` of its total debt from the settle order, then reevaluate the debt position list and loop.

This would bring better experience for debtors.

Note: the design is incomplete, there are edge cases to consider to avoid infinity loop and for better performance.