2021-02-22 bitUSD parameters adjustment

[abit]

Purpose:
to improve market liquidity.

Changes:
ICR (Initial collateral ratio) adjusted from 210% to 240%
Maximum supply adjusted from 1,500,000 bitUSD to 1,800,000 bitUSD
Added  zapata42-witness to price feed producers

Current parameters:

ICR (Initial collateral ratio) 240%
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:
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
1.2.987999  zapata42-witness
1.2.1035733 syalon

Borrowing is enabled. The maximum supply is limited to 1,800,000 bitUSD.
Force-settlement is enabled.