file stores keep children that mysql deletes with the parent

[aberoham]

Destroy a group on json or toml and its nameservers stay behind. Same for a
user's sessions and a zone's records. mysql removes all three, because it has a
constraint and the file stores have nothing.

Measured on 96f4799 through lib/<entity>/index.js, one fixture per pair:

destroy	child	constraint	mysql	json	toml
group	nameserver	nt_nameserver_ibfk_1	gone	stays	stays
user	session	nt_user_session_ibfk_1	gone	stays	stays
zone	zone_record	nt_zone_record_ibfk_1	gone	stays	stays

The file-store destroy() is the whole story:

  async destroy(args) {
    const groups = await this._load()
    const before = groups.length
    const filtered = groups.filter((g) => g.id !== args.id)
    if (filtered.length === before) return false
    await this._save(filtered)
    return true
  }

user and zone are the same method over a different array.

Repro on json:

const Group = (await import('./lib/group/index.js')).default
const Nameserver = (await import('./lib/nameserver/index.js')).default

await Group.create({ id: 47101, parent_gid: 0, name: 'refgraph.test' })
await Nameserver.create({
  id: 47104, gid: 47101, name: 'ns.refgraph.test.',
  address: '203.0.113.10', type: 'nsd', ttl: 3600,
})
await Group.destroy({ id: 47101 })

;(await Nameserver.get({ id: 47104 })).length // 1 on json and toml, 0 on mysql

Watch how you check it. mysql reads a session through its user
(session/store/mysql.js joins nt_user on u.deleted=0), so a session that
survived still reads as absent once the user is gone. Delete the row a second
time and see whether anything was there.

#61 adds a fourth, measured the same way: destroy a group and its delegation
rows stay on json and toml, while nt_delegate_ibfk_1 removes them on mysql.
That one hides too — the file store's _present() filters on live group names.

group -> zone and group -> user keep the child on every store. Neither has a
constraint behind it, so that looks like a decision to make, not a difference to
fix.

[msimerson]

Skip creating the issue, put all the content in the PR.