Margin call orders: call price should change accordingly when partially called

[abitmore]

With 0.9.x market engine, test case:

• When feed price is 1 BTA/BTS, Alice shorts 5000 BTA at feed price,
  • BTA owed is 5000 BTA
  • collateral is 10000 + 5000 = 15000 BTS
  • collateral rate is 15000 * 1 / 5000 = 300%
  • call price is 2 * 5000 / 15000 = 0.67 BTA/BTS
• When feed price drop to 0.6 BTA/BTS
  • collateral rate drop to 15000 * 0.6 / 5000 = 180%
• Alice's order get margin called partially, for example 3000 BTA is covered
  • BTS paid from collateral is 3000 / 0.6 = 5000 BTS
  • BTA owed is updated to 5000 - 3000 = 2000 BTA
  • remain collateral is 15000 - 5000 = 10000 BTS
  • collateral rate is 10000 * 0.6 / 2000 = 300%
  • call price should change to 2 * 2000 / 10000 = 0.4 BTA/BTS, and the order shouldn't be called again soon (it's unchanged in 0.9.0/0.9.1 thus is possible to be called again in next block)