Jetpack Protect: optimize transient clean up

#### Description
Huge number of outdated Jetpack Protect transients may affect the cleanup process, or even bring down the whole website.

##### This may happen if
- WP-cron is not functioning properly, and the transients don't get removed, which may slow down the website.
- WP-cron did not function properly, and once it started working, it tries to remove too many transients at once, bringing down the website.

##### Transient keys
`_transient_timeout_jpp_li_*`
`_transient_jpp_li_*`

##### The code
https://github.com/Automattic/jetpack/blob/master/modules/protect/transient-cleanup.php

#### To Do
1. Modify the Mocker tool to create random transients in Jetpack Protect format.
2. Review the cleanup process and optimize it if possible.
3. Add a timeout to confirm that the website keeps working even if there are too many transients to delete at once.