USC Testnet v.0.0.6

Updated difficultyBondDivisor.
Updated difficulty calculation algorithm.
Updated block generation time.

----Testnet fork time----
Thursday, August 16, 2018 3:00:00 PM GMT+08:00
Friday, August 17, 2018 10:00:00 AM GMT+08:00
Monday, August 20, 2018 11:00:00 AM GMT+08:00