Design checkpoint block on every epoch

[ngtuna]

In each epoch, there will be checkpoint block created for these below actions:

• formalize reward transactions
• update set of validators
• formalize new order of validators for next epoch

[ngtuna]

Step by step:

Calculate reward/slash:

1) calculate reward by signatures 2) calculate slash if it is needed

Set of new masternodes:

1) collect seed matrix 2) update set of new masternodes (validators) for next epoch 3) calculate pair <m1,m2>

[ngtuna]

Following actions must be executed at checkpoint block:

• calculate reward: by sending direct call to Signing SMC: getTotalSign()
• get new set of masternodes: by sending direct call to Voting SMC: getM1()
• get new set of m2 (index only): by sending direct call to Random SMC: getM2()

[ngtuna]

Done via couple of PRs. Checkpoint block design is clear for now.