Consensus: change block reward

[semuxgo]

Rational

• To avoid network instability caused by the sudden removal of block rewards
• Motivated by existing blockchain coin supply strategy, like bitcoin
• The max supply will be kept the same at 100M.

Block        1 - 10000000 : 3 SEM
Block 10000001 - 25000000 : 2 SEM
Block 25000001 - 40000000 : 1 SEM
Block 40000001 -          : 0 SEM

[lodyman93]

why don't we use block reward constant halving
or maybe percentage dropping this way the supply will last till the tx fees are the enough incentive for the validators