NEXUS-5472: Concurrency issues with parallel updates

As bamboo shows, there is a concurrency problem when two
proxies are being updated, and both are member of same group.

The propagation of one proxy took others WL prefix file, that was
in process of validation and puked with NPE as validation fails.

Now this case is excluded.
Improving ITs

Current ITs seems unstable due to that they are unable to properly
detect when and what async operation is done on server. To improve
that introduced new method that listens for publishing too, as
previous failures were due to central being discovered, but public
group prefix file was still being built, and asserts were failing.