Thread safety issue in PartyMerger

[jmderuty]

{"message":"An error occurred while running the party merger default.","data":{"content":{"location":"scenes/partyMerger/partyMerger-default#0","context":{"template":"partyMerger","sceneId":"partyMerger-default","shardId":0},"message":"An error occurred while running the party merger default.","data":{"Type":"InvalidOperationException","Message":"Collection was modified; enumeration operation may not execute.","Frames":[" at System.Collections.Generic.Dictionary2.Enumerator.MoveNext()"," at System.Threading.Tasks.Task.WhenAll[TResult](IEnumerable1 tasks)"," at Stormancer.Server.Plugins.PartyMerging.PartyMergingService.Merge(CancellationToken cancellationToken) in /_/src/Stormancer.Plugins/Party/Stormancer.Server.Plugins.PartyMerging/PartyMergingService.cs:line 255"," at Stormancer.Server.Plugins.PartyMerging.PartyMergingState.<>c__DisplayClass14_0.<<-ctor>b__0>d.MoveNext() in /_/src/Stormancer.Plugins/Party/Stormancer.Server.Plugins.PartyMerging/PartyMergingService.cs:line 62"],"InnerExceptions":[]}},"app":"/darktales/live-1-0/8408abaa-ab74-4c7b-9490-31d54368f24c/26?"}}