Always allow updating a call order to higher collateral ratio

[abitmore]

Current behavior: if a call order's collateral ratio is below MCR but it's not filled immediately due to no enough buy orders, it will hang there. The owner can adjust its collateral ratio only if to close the position, or the new ratio is above MCR, or the call order get completely filled (margin called). While this prevents the owner from maliciously reducing collateral ratio (to increase possibility of black swan event), it also prevents the owner from slightly increasing the collateral ratio (to decrease possibility of black swan event). IMHO the latter should be allowed.

Code:

      // check to see if the order needs to be margin called now, but don't allow black swans and require there to be
      // limit orders available that could be used to fill the order.
      if( d.check_call_orders( *_debt_asset, false ) )
      {
         const auto call_obj  = d.find(call_order_id);
         // if we filled at least one call order, we are OK if we totally filled.
         GRAPHENE_ASSERT(
            !call_obj,
            call_order_update_unfilled_margin_call,
            "Updating call order would trigger a margin call that cannot be fully filled",
            ("a", ~call_obj->call_price )("b", _bitasset_data->current_feed.settlement_price)
            );
      }
      else
      {
         const auto call_obj  = d.find(call_order_id);
         FC_ASSERT( call_obj, "no margin call was executed and yet the call object was deleted" );
         //edump( (~call_obj->call_price) ("<")( _bitasset_data->current_feed.settlement_price) );
         // We didn't fill any call orders.  This may be because we
         // aren't in margin call territory, or it may be because there
         // were no matching orders.  In the latter case, we throw.
         GRAPHENE_ASSERT(
            ~call_obj->call_price < _bitasset_data->current_feed.settlement_price,
            call_order_update_unfilled_margin_call,
            "Updating call order would trigger a margin call that cannot be fully filled",
            ("a", ~call_obj->call_price )("b", _bitasset_data->current_feed.settlement_price)
            );
      }
   }

I think this is a bug so don't need a BSIP to fix it.

[pmconrad]

Makes sense.

[abitmore]

PR #655 merged. Closing.