Multiple sockets for opcache

[schmengler]

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• [ ] Bug fix #…?
• [X] New feature?
• [ ] BC breaks?
• [ ] Tests added?
• [X] Docs added?

This PR extends the cachetool configuration, so that multiple sockets (e.g. for multiple domains) can be addressed. See updated documentation for details. A retry mechanism has been added because clearing the opcache for multiple sockets too fast can result in errors.

Also downloading the cachetool has been changed analogous to composer, to the .dep directory

For backwards compatibility:

• the old cachetool configuration still works and has precedence, albeit undocumented
• if cachetool.phar is in the current/release path,it will be used