[7.9] Synchronize updates to the authorization filter

A stackdump from a production environment experiencing a hiccup revealed that updates to the authorization filter may be happening with high concurrency (22 threads updating the same filter at the same time). This scenario can occur when there are many searches going on in paralel by the same user while part of the filter needs to be updated. For instance a siteuser. While the index is being updated executing a search on the segment reader is blocked. During this time many threads may start queuing to update the filter. Once the index is unblocked all threads start to simultaneously execute the authorization query on the segment reader. It is better to synchronize such updates to the authorization filter.