stablerange: move the subrange cache into our new class This is the first step toward having more logic in that class

depth: extract code dedicated to depth of a merge in its own function The merge case is more complicated than the regular one, we extract is for the sake of clarity.

stablerange: move 'depth' inside a new 'stablerange' module The stablerange used for discovery requires significant amount of code. There is algorithme, cache, cache persistence, etc. So we create a dedicated module for it. The function is directly moved into a rich class handling cache (for now in memory) because we know we will need it.