Reduce shard staking requirement for PoSW

[ninjaahhh]

as our block reward drop comes near (factor = 0.88), we should also reduce PoSW staking requirement accordingly on shards.

[ninjaahhh]

this would be a consensus change cc @pingke @qizhou

also the targeting shard height for block reward reduction is 3153600, as of now (10:38am 3/30 PT) we are at around 2950000, so roughly about 28 days to go

[qizhou]

Observed reduced staking