Lighthouse: Possible consensus bug in `process_exits`

[JustinDrake]

The process_exits function verifies the voluntary exits in parallel. If there is a duplicate voluntary exit then

    verify!(
        validator.exit_epoch == spec.far_future_epoch,
        ExitInvalid::AlreadyExited(exit.validator_index)
    );

will not trigger, allowing for voluntary exists to be initiated multiple times. The spec requires voluntary exits to not have been initiated.

    # Verify exit has not been initiated
    assert validator.exit_epoch == FAR_FUTURE_EPOCH

(It seems initiate_validator_exit does not provide protection by silently returning early if the validator already initiated an exit.)

[michaelsproul]

Another good catch. I think this is confirmation that we should go through an remove all the parallel iterators from block processing.

[michaelsproul]

I'm working on fixing this, and improving our block processing tests in the process. I'm going to see if I can improve the ergonomics of the test builders, which have been long considered quite unwieldy :crossed_fingers: