General framework for new repository formats.

The repository format has been fixed at 5 since 1.00. It was a compile-time constant written into every info file and manifest, and any mismatch at load was a hard error. A version could only read the format it wrote, so introducing a format meant a flag day where every repository had to be recreated. 1.00 was that flag day and this branch is so the next format does not need one.

A file is readable when its format is between REPOSITORY_FORMAT_MIN and REPOSITORY_FORMAT_MAX. A version can read a format it does not write and a repository can hold both while older backups and archives expire.

The format is requested with repo-format, which is repo indexed and valid only for stanza-create and stanza-upgrade. It is command line only so a format left in the configuration cannot migrate a stanza as a side effect of an upgrade run for another reason, such as a PostgreSQL version upgrade. The default stays 5.

stanza-upgrade --repo1-format=6 rewrites the two info files and nothing else, so a repository of any size migrates in the time it takes to write them. A downgrade is refused, since the info files are what stop a version from reading a format it does not support. The two files are saved separately, so an upgrade interrupted between them leaves a mismatch that running the upgrade again repairs and that every command reading both files now reports.

A backup set adopts a new format only at a full backup. A prior backup is a candidate for diff or incr only if it is at the format new backups are written with, and a resumable backup at another format is discarded, so a backup set is never mixed and its format can be read once rather than per file. Migrating therefore costs one full backup per stanza.

Once the info files are at format 6 a version that supports only format 5 cannot read the stanza at all, including the backups still at format 5. Such a version fails at info load with "expected format 5 but found 6" before touching any data.