[Bug]: Dirty DB reads on `\OC\BackgroundJob\JobList::add`

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

There are dirty db reads that may have an impact on clustered db setups when adding multilple jobs (\OC\BackgroundJob\JobList::add) of the same type.

Steps to reproduce

1. Register multiple jobs like \OCA\Mail\Service\AccountService::save

Can cause dirty reads and addition of multiple same jobs

Expected behavior

No dirty readys of the db.

Installation method

None

Nextcloud Server version

master

Operating system

None

PHP engine version

None

Web server

None

Database engine version

None

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

Found in https://github.com/nextcloud/groupware/issues/24

Start by tracing \OC\BackgroundJob\JobList::add, using the multiple registrations described for \OCA\Mail\Service\AccountService::save as the reproduction case. Investigate the database reads involved and verify that adding jobs of the same type does not produce dirty reads or duplicate jobs, especially on clustered database setups.