Implement self-stake slashing for double producing blocks

[koseoyoung]

Along with already implemented first slashing which is probation for unproductive delegates, we have to design the second slashing mechanism in order to punish double producing delegates.

1. We need a detection mechanism that detects submitted blocks which have different block hash by the same producers at the same height.

2. Once detected, such delegate's self-stake should be confiscated depending on predefined percentage or amount. 2-1. should also put on probation status 2-2. need to decide the exact number for percentage or amount

[raullenchai]

cc @CoderZhi

[raullenchai]

More realistically, we could implement the detection part first.

[raullenchai]

per @CoderZhi, here are the steps:

• [x] Make sure our code won't double sign
• [ ] Detection of DS
• [ ] Report of DS to the protocol
• [ ] Slash double-signers
• [ ] PR'd but not right way