* GetByHeight will now switch to using binary search once
we have >=50 items.
* Feedback from @ebuchman to catch a missed spot where
we forgot about lazy sorting that the original code
assumed would always have sorted commits by height.
Added a lazy sorting routine here too.
A test as well to ensure that we always get the properly
sorted and last value.
Addressing PR review requests from @melekes and @ebuchman to
add a test that checks that the heights returned from both are
the same thus providing a perceptible equivalence of the code
linear range search vs binary range search code.
The certifier package is renamed to light. This is more descriptive
especially in the wider blockchain context. Moreover we are building
light-clients using the light package.
This also adds godocs to all exported functions.
Furthermore it introduces some extra error handling. I've added one TODO
where I would like someone else's opinion on how to handle the error.