[consensus] Remove unnecessary Arc wrapper on futures_locks::RwLock
[consensus] Remove unnecessary Arc wrapper on futures_locks::RwLock

future_locks::RwLock is already an Arc internally. This removes the extra,
unnecessary wrapper. See the docs at
https://docs.rs/futures-locks/0.3.3/futures_locks/struct.RwLock.html

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