Fixed issue with SortedSet not being emptied in Core.Util.Fst.Util, and added thread safety, since SortedSet doesn't have the necessary atomic operations.
Fixed issue with SortedSet not being emptied in Core.Util.Fst.Util, and added thread safety, since SortedSet doesn't have the necessary atomic operations.

NightOwl888/lucenenet,NightOwl888/lucenenet,jeme/lucenenet,apache/lucenenet,sisve/lucenenet,NightOwl888/lucenenet,NightOwl888/lucenenet,jeme/lucenenet,laimis/lucenenet,jeme/lucenenet,sisve/lucenenet,laimis/lucenenet,apache/lucenenet,apache/lucenenet,jeme/lucenenet,apache/lucenenet