[Feature] Update block difficulty target time to 20 seconds

[howardwu]

🚀 Feature

testnet1 currently has 10 seconds baked as its target time, however as the network is producing blocks at 30 seconds, we have never truly tested difficulty adjustments in prod, only in development.

For testnet2, we should be setting the difficulty target time to 20 seconds, and adding sanity checks that these updates occur correctly in the block header and consensus protocol.

Implementation

Are you willing to open a pull request? (See CONTRIBUTING)