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 smallintlazy 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)