stablerange: clarify sql transaction logic This should help concurrent access.

stablerange: use mergepoint based algorithm for the official stable range Changing the official stable range will impact all users of the infrastructure. We update version number of the cache file and discovery methods to clarify this. We do no keep compatibility with the old method over the wire. The new algorithm is much better and keeping compat is more work than we have time for right now.

stablerange: explicitly create a stablerange entry for branchpoint method We are about to change the default.

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.