baseset: drop custom __add__ method This add method is enforcing non-laziness, disabling multiple optimisations. Benchmarks do not spot any significant differences but real usecase may. This will also be important for further improvements to addset later in this series.

obsolete: use format version 1 as the default for obsstore

test-obsolete: remove subminute timezone in test Obsmarker format "1" does not supports sub minute timezone. So we change the test to something slightly more sensible.

obsolete: add a "format.obsstore-version" config option This option controls what version of the binary format to use when creating a new obsstore file. Default is still the old format. No safeguards are currently placed around the option value, but no clueless users are in danger of harm since it is undocumented.