runes: Use next id from runes table not runes_uniqueid from vars

[ShahanaFarooqui]

Changelog-Fixed: runes: use runes table id instead runes_uniqueid from vars because it returns incorrect unique id if rune/s migrated from datastore.

Issue (#6696):
The issue occurs when user saves the rune (created by commando-rune) in the datastore with v23.02 and then upgrades the DB with v23.08. This upgrade calls migrate_datastore_commando_runes function to migrate runes from datastore to runes table. But it does not increment runes_uniqueid of vars table. So when user creates a new rune, CLN gets old next_unique_id and fails with UNIQUE_ID constraint error.
This issue effects Umbrel's users and users who saved their runes from older version in the datastore.

Solution 1:

• Add another migration function to recalculate runes_uniqueid and update vars table.
• Update runes_uniqueid in vars table in migrate_datastore_commando_runes for users who are yet to update.

Solution 2:

• Do not use runes_uniqueid, instead calculate the next_unique_id from runes table.
• id can be auto generated for runes insert but we still need to maintain it in runes because:
  - It is also required by rune_derive_start
  - counter starts from 1 (not 0) if not explicitly passed to the query.

Implemented solution 2 as it is clean and future proof.

[rustyrussell]

According to the changelog lines and lack of tests, this doesn't change anything? :)

The original purpose of the var was that we can have missing runes. Perhaps that's a premature optimization, though. If so, you should delete the now-unused variable.

[ShahanaFarooqui]

According to the changelog lines and lack of tests, this doesn't change anything? :)

Updated changelog from None to Fixed. Also copied bug details from the original issue to describe the problem.

The original purpose of the var was that we can have missing runes. Perhaps that's a premature optimization, though. If so, you should delete the now-unused variable.

Deleted unused variable.

[rustyrussell]

Ack 63fc378