Allow tests to report/fail on many ZK watches being parallelly requested on the same data

Initial patch for  SOLR-6370, starts off with conservative defaults (just show report at shutdown), but could be made stricter as we fix up issues this uncovers (there are lots which do show up).

I opted to implement this on the server side so that the real {{SolrZkClient}} doesn't have to be modified for tests, and all tests are opted in by default.