lite2: modify TrustedValidatorSet func to return height

[melekes]

If we're going to allow 0 height as an argument, returning height as a second param seems reasonable.

TrustedValidatorSet(0) => (*ValidatorSet, 10, nil)
TrustedValidatorSet(9) => (*ValidatorSet, 9, nil)

[melekes]

@cmwaters what do you think?

[cmwaters]

At the moment, TrustedValidatorSet doesn't have the behaviour where if you pass 0 as the height it will return the last validator set. Actually I modified it in callum/verify-bakwads so that it doesn't - will need to revert that then if we want this behaviour.

Anyway, this functionality is only useful if we want to know the latest validator set height (else we already have the height saved as input). Can't we just call c.LastTrustedHeight()

[melekes]

We can, but then the user will be forced to do

	latestHeight, err := c.LastTrustedHeight()
	if err != nil {
		return nil, err
	}
	if latestHeight == -1 {
		return nil, errors.New("no headers exist")
	}
	if height > latestHeight {
		return nil, errors.Errorf("unverified header requested (latest: %d)", latestHeight)
	}
	if height == 0 {
		height = latestHeight
	}

by herself. Don't you think it's bad?