Fix EZP-23168: ESI Request URLs exceed acceptable size of 8KB

> https://jira.ez.no/browse/EZP-23168

In the ESI Request URL information about ALL site accesses is transmitted. With a larger number of site accesses the URL exceeds the size of 8192 Bytes which most servers accept. As a result, the ESI call fails.

This fix inhibits the serialization of the site access map.

It works in our case, but I'm not sure whether there are any limitations or side effects.
This is not really meaningful for someone reading the phpdoc. Better explain what you do instead, and add `@see https://jira.ez.no/browse/EZP-23168`