Add stake mining

[spartucus]

This feature change mining rules from POW to POW + POS. Miners are encouraged to stake more coin, more time to get POS reward.

Here are details:

1. Miner locked_transfer to himself(herself) with at least 1 XMC, 1 day.
2. The max balance is 300'000 XMC, the max lock time is 12 month.
3. POW reward is 10%(exclude fees) of coinbase reward, max POS reward is 20% of coinbase reward, 70% of coinbase reward is belong to the Monero-classic foundation.