Change Authority Discovery in a better way

[badkk]

When I using BABE, I found the blocking time was unstable because of the authority peers sometimes be disconnected, How can I make sure the authorities always connected each other?

BABE + AUDI can solve this problem, however, authority_discovery now is in experimental stage, follow polka's change in service.rs code/update substrate in stable 2.0.0 version

[badkk]

already fixed by paritytech/polkadot#624