Make validator shares transferable to allow for key rotation

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Thanks @jim for the proposal. Great idea to disable transferFrom/approve. Now that I think about it, it’s not meant to be traded in most cases and hard to trade due to liquidity.

Would you be okay doing PR on branch release-0.3-3 (https://github.com/maticnetwork/contracts/tree/release-0.3-3)? Just editing ERC20NonTransferable should it.

Thanks a lot!