Cosmos-sdk: Slashing Period -> Tombstone

[rigelrozanski]

It's possible that we may be able to remove the concept of slashing period if we introduce the concept of "killing" a validator if it performs a serious malicious act such as a double sign. Under this situation the validator could be pushed out of the validator set with no opportunity of coming back and forcing all the delegations out.

This should be considered post-launch, however as per conversations we may we to do prelaunch if we decided to switch from active slashing (after being informed from https://github.com/cosmos/cosmos-sdk/issues/2362)

[ValarDragon]

I think this would simplify the logic alot, so I'm very much in favor of it. @cwgoes also suggested the name "exiling" for this. I think thats both more accurate, and a better name for this!

[rigelrozanski]

This is where validators go when they've been bad

[jackzampolin]

This will be closed once https://github.com/cosmos/cosmos-sdk/pull/3225 is merged

[rigelrozanski]

rather than commenting here would have been more beneficial to add "closes #2363" to the description comment of that PR (you can edit others PR description main comments) ... doing that now