sparse-revlog: rework the way we enforce chunk size limit
We move from a O(N) algorithm to a O(log(N)) algorithm.
The previous algorithm was traversing the whole delta chain, looking for the
exact point where it became too big. This would result in most of the delta
chain to be traversed.
Instead, we now use a "binary" approach, slicing the chain in two until we
have a chunk of the appropriate size.
We still keep the previous algorithm for the snapshots part. There are few of
them and they are large bits of data distant from each other. So the previous
algorithm should work well in that case.
To take a practical example of restoring manifest revision '59547c40bc4c' for
a reference NetBeans repository (using sparse-revlog). The media time of the
step slice-sparse-chain of perfrevlogrevision improve from 1.109 ms to