improve index on appconfig

[ArtificialOwl]

follow-up of #42820

• only create new fields if not existing yet
• removing appconfig_configkey_key index on single field configkey as there is no search on this field
• redefine lazy as Types::INTEGER (smallint(6)) instead of Types::BOOLEAN to be able to set it a NotNull (set in 28-rc2)
• index on lazy
• removing the use of $migrationCompleted that was needed during the migration from 28 to 29 but not anymore since we generate the (previously) missing fields lazy and type in 28
• do not select lazy during loadConfig as we already know its value

Note: After some discussion, it seems better to switch lazy to INTEGER so the field can be set as NotNull (because Oracle)

Fixes #43103

[ArtificialOwl]

I have estimated that a full squash+rebase was a lot better as I reverted a lot of code:

• upstream of [stable28] prepare migration to lazy config #42820 to master
• commenting most of the code from previous migration step Version28000Date20231126110901 (+ explication)
• commenting most of the code from previous migration step Version28000Date20231126110901 (+ explication)
• new migration step Version29000Date20240124132201 to remove previous boolean lazy field and related indexes, fix type to unsigned and remove useless index on configkey.
• new migration step Version29000Date20240124132202 to create a smallint lazy field.
• keep migration fall back on reading app config values to keep upgrade compatibility with versions prior to 28.0.2
• switching write/read request to lazy field from managing a boolean to managing a smallint
• remove lazy from select list if known when calling loadConfig()

---

tested on

• fresh install
• upgrade from 28.0.2-rc2
• upgrade from 28.0.1 (only issue doing this is that writing (new/edit) app config value will throw sql exception

---

final result on each test:

MariaDB [nextcloud_nc29]> DESCRIBE oc_appconfig; SHOW INDEX FROM oc_appconfig;
+-------------+----------------------+------+-----+---------+-------+
| Field       | Type                 | Null | Key | Default | Extra |
+-------------+----------------------+------+-----+---------+-------+
| appid       | varchar(32)          | NO   | PRI |         |       |
| configkey   | varchar(64)          | NO   | PRI |         |       |
| configvalue | longtext             | YES  |     | NULL    |       |
| type        | int(10) unsigned     | NO   |     | 2       |       |
| lazy        | smallint(5) unsigned | NO   | MUL | 0       |       |
+-------------+----------------------+------+-----+---------+-------+
5 rows in set (0.002 sec)

+--------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+
| Table        | Non_unique | Key_name  | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Ignored |
+--------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+
| oc_appconfig |          0 | PRIMARY   |            1 | appid       | A         |         142 |     NULL | NULL   |      | BTREE      |         |               | NO      |
| oc_appconfig |          0 | PRIMARY   |            2 | configkey   | A         |         142 |     NULL | NULL   |      | BTREE      |         |               | NO      |
| oc_appconfig |          1 | ac_lazy_i |            1 | lazy        | A         |           2 |     NULL | NULL   |      | BTREE      |         |               | NO      |
+--------------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+
3 rows in set (0.001 sec)