Implement Staking module

[HoOngEe]

I tried to simulate staking logic previously is operating inside the host.
If it is merged into mold, the following tasks should be carried out.

1. Remove staking logic from the host.
2. Modify TopLevelState to store only CompactValidatorSet which contains only validators' public key and abstract voting powers.
3. Remove parameters only used by Staking.

Followings are not implemented yet:

1. Executing ReportDoubleVote and banning malicious validators.
2. Creating transactions from higher level input (e.g. graphQL mutation request)

[HoOngEe]

Details will be modified after coordinator - module interface is fixed.

[HoOngEe]

@sgkim126 @majecty
I checked all the modifications in the pr #388. Before applying the changes in staking module, I have two concerns.

1. transaction_index in a "block" could not be resolved from the module level. I naively thought that adding transaction_index when executing transactions based on the index in staking-transactions. However, it also produces another complexity. We're now using an interface function prepare_transactions which includes transaction executions to determine post-processing transactions(previously implicit) based on the state after execution. After the post-processing transactions are included, the index will be changed so the index when creating a block and the index after being delivered are different. Manually ignoring post-processing transactions when calculating the index is a possible way but unnatural.
2. Host-side validator ordering is important because it matches with the bitset in headers. In a modules system a host-side validator only stores a public key and an abstract voting power. To follow the new rule, host should additionally stores nominated_at_block_number and nominated_at_transaction_index. I don't think it's a general requirements for modules to determine those things as a tie-breaker.

[HoOngEe]

After a discussion @majecty
I realized I misunderstood the boundary the second is no more a problem. The norm of host-side ordering is public-key so only staking module is entirely responsible for tie-breaking.
For the first one, I'll rename the variables and apply the change.