[consensus] try to simplify RecoveryData::find_root()
[consensus] try to simplify RecoveryData::find_root()

- min_by_key should help make the invariant more clear
- in case of round equality, the choice changes (but equality should not happen anyway?)

Closes: #1018
Approved by: zekun000

aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core