2021-2-9 bitUSD revival and new parameters

[abit]

bitUSD is now revived.

Current parameters:

ICR (Initial collateral ratio) 210%
MCR (Maintenance collateral ratio) 150%
MSSR (Maximum short-squeeze ratio) 105%
MCFR (Margin call fee rate) 3%
FSFP (Force settlement fee percent) 3%
FSO (Force settlement price offset) 2%
Force settlement delay 24 hour
Maximum force settlement volume per hour 0.5%
Price feed threshold 0 (temporary)
Minimum feeds 3 (temporary)
Price feed producers list (7): (temporary)
1.2.183     hiquanta
1.2.14035   xn-delegate
1.2.35248   in.abit
1.2.389463  nexus01 (Ross Walker)
1.2.420186  clone (yexiao)
1.2.738675  xbtsio-wallet
1.2.1035733 syalon

Borrowing is enabled. The maximum supply is limited to 1,500,000 bitUSD.
Force-settlement is enabled.