[BUGFIX] slow response when allowedSites = __all

[netcoop]

What this pr does

Fixes very slow response when search.query.allowedSites = __all, esp. on multi-site installations.

How to test

Add a couple of extra sites with their own domain or subdomain and measure the time it takes for the method getDomainListOfAllSites to complete. Without the patch, this could easily run up to 0.3 or 0.5 seconds for each domain. With this fix, the time is negligible (and so it should, because this method is process uncached).

Fixes: #3292