v0.6.0

The admin service administers a migrator running several migration sets, such as a library's
shipped set beneath the consumer's own. The first such set is blobfs's.

Changed

• The sqlate requirement is v0.4.0, whose migrator runs one or more migrate.Sets, each over
  its own history table.
• Breaking: admin.Status is {Ready, Sets}: one admin.SetStatus per set, in declared
  order, with its name, history table, head, latest version, dirty mark, pending versions, and
  migrations. Ready is true when every set is clean and complete. A dirty set's pending
  versions are the ones above its dirty head; the single-set status reported none.
• Service.Status returns the migrator's error, naming the set, when a history carries a row
  its set does not, and clears Ready. It reported that history as not ready before.
• Breaking: Service.Down, Service.Steps, and Service.Force take the name of the set
  they act on. Force(ctx, set, version) is the repair for a dirty set.
• Service.Reset reverts every set, the last declared first, dropping each history table, then
  applies every set and seeds the named state. It reverted only the one set before.
• Service.Start logs the pending migrations set by set before it applies them, and each set's
  head once current.

Added

• admin.ErrUnknownSet, returned before any I/O when a verb names no set or a set the migrator
  does not run.
• admin.SetStatus.