core: improve the network stability when double sign happens

[buddh0]

Description

core: improve the network stability when double sign happens

Rationale

for example, 3 validator(vb, vc, vd) do an double sign attack together,
they can easily spilt the network into 8 subnets, then the whole chain stops

to solve this issue, we should give a definited rule to select forks.

in additon, this change is back compatible

Example

add an example CLI or API response...

Changes

Notable changes:

• add each change in a bullet point here
• ...

[zzzckck]

I think we can improve this expression later, to make it easier to read.