stablerange: be more cautious when deleting the sql connection This triggered some crash in further development. This also match what is done elsewhere with this attribute.

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.