lite: verify all headers

[ebuchman]

We currently use bisection to sync a lite client so we don't have to verify all headers. But this unsafe. It looks like doing bisection safely may require application specific components: #3244.

As an immediate remediation, we should verify all headers in order.

[jaekwon]

See #3244 (comment) on why we need to implement counterfactual signing anyways.

If +2/3 are signing something bad, we've got bigger problems... this solution isn't necessary.

[liamsi]

Should we close this then?

[tac0turtle]

with the work of #3906, #3847, #3911 this issue can be closed.