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.

obsolete: introduce a new binary encoding for obsmarkers (version 1) This new encoding explicitly stores the date and parents allowing a significantly faster marker decoding. See inline documentation for details. This format is not yet used to store format on disk. But it will be used in bundle2 exchange if both side support it. Support for on-disk format is coming in other changesets.

obsstore: add a flag for sha256 hashes We add flag to inform that the marker is using sha256 hashes. As format 0 is not able to handle sha256 hashes (32 bytes long), we plain crash if we even attempt to encode a sha256 with it.

obsolete: use uint## in the format documention This is shorter and kind of more readable for people who care about binary format.

obsolete: gather _fm0 meta encoding with other _fm0 code

obsolete: _rename decodemeta to _fm0decodemeta This will be format zero specific.

obsolete: _rename encodemeta to _fm0encodemeta This will be format zero specific.

obsolete: store metadata as a tuple of (key, value) pairs (API) Different formats will encode metadata in different ways. So we cannot keep the binary blob in the object anymore. We use a tuple to ensure it is immutable and hashable.