Upgrade Validator Manager

Convert PoA to PoS

A PoAValidatorManager can later be converted to a PoSValidatorManager by upgrading the implementation contract pointed to by the proxy. After performing the upgrade, the PoSValidatorManager contract should be initialized by calling initialize as described above. The validator set contained in the PoAValidatorManager will be tracked by the PoSValidatorManager after the upgrade, but these validators will neither be eligible to stake and earn staking rewards, nor support delegation.