stablerange: have a mergepoint based sql backed class Everything is so generic that getting a class is 3 line of codes.

stablerange: introduce a sql backed version of the generic on disk cache This is getting use close to actually using the new sorting and algorithm for real.

stablerange: add a base class for on disk stored stablerange class

stablesort: use on disk cache for test

caches: factorise the cache warming check The old conditional was duplicated and hard to read. We factorise everything into the `utility` module.

stablerange: use repo-carried stablesortcache That one is common to all and disk persisted

stablesort: expose the cache through the repository Let have a unique instance, keep it warm and accessible.

stablesort: implement an ondisk cache Persisting the cache on disk will be important for large repositories.

stablesort: realign a misaligned continue We should make a better use of the jump cache now.

stablesort: remove some dead code This closure is no longer in use.

stablesort: abstract all cache access This prepare an on disk version of the cache

stablerange: use first merge cache to skip over linear section If we are to skip over a large section of the range, skipping everything until the first merge seems like a good idea.

stablerange: add a new 'firstmerge' cache This cache store the first merge ancestors of a changesets. This is useful to avoid iteration over the changelog when building stablerange.

stablerange: drop unused `until` utility We no longer needs this function.

stablerange: use cached size data instead of walking the graph Less iteration is better.

stablesort: record previous segment size in the jump That was, we can skip over a full jump without walking it. We'll make use of this in the next changeset.