When deleting messages, match by message id and seq no.

[aelfsyg]

It is possible that service bus messages will share a message id. This was causing issues when deleting messages as it called .Single(pred) yet there were multiple matching messages. This PR matches on message id and sequence number when deleting messages instead.

System.InvalidOperationException: Sequence contains more than one matching element
   at System.Linq.ThrowHelper.ThrowMoreThanOneMatchException()
   at System.Linq.Enumerable.TryGetSingle[TSource](IEnumerable`1 source, Func`2 predicate, Boolean& found)
   at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source, Func`2 predicate)
   at PurpleExplorer.Models.MessageCollection.RemoveMessage(String messageId) in /home/ae/src/ext/PurpleExplorer/PurpleExplorer/Models/MessageCollection.cs:line 53
   at PurpleExplorer.ViewModels.MessageDetailsWindowViewModel.DeleteMessage(Window window) in /home/ae/src/ext/PurpleExplorer/PurpleExplorer/ViewModels/MessageDetailsWindowViewModel.cs:line 83
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__128_0(Object state)
   at Avalonia.Threading.AvaloniaSynchronizationContext.<>c__DisplayClass5_0.<Post>b__0() in /_/src/Avalonia.Base/Threading/AvaloniaSynchronizationContext.cs:line 33
   at Avalonia.Threading.JobRunner.Job.Avalonia.Threading.JobRunner.IJob.Run() in /_/src/Avalonia.Base/Threading/JobRunner.cs:line 181
   at Avalonia.Threading.JobRunner.RunJobs(Nullable`1 priority) in /_/src/Avalonia.Base/Threading/JobRunner.cs:line 37
   at Avalonia.X11.X11PlatformThreading.CheckSignaled() in /_/src/Avalonia.X11/X11PlatformThreading.cs:line 164
   at Avalonia.X11.X11PlatformThreading.RunLoop(CancellationToken cancellationToken) in /_/src/Avalonia.X11/X11PlatformThreading.cs:line 244
   at Avalonia.Threading.Dispatcher.MainLoop(CancellationToken cancellationToken) in /_/src/Avalonia.Base/Threading/Dispatcher.cs:line 65
   at Avalonia.Controls.ApplicationLifetimes.ClassicDesktopStyleApplicationLifetime.Start(String[] args) in /_/src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs:line 120
   at Avalonia.ClassicDesktopStyleApplicationLifetimeExtensions.StartWithClassicDesktopLifetime[T](T builder, String[] args, ShutdownMode shutdownMode) in /_/src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs:line 209
   at PurpleExplorer.Program.Main(String[] args) in /home/ae/src/ext/PurpleExplorer/PurpleExplorer/Program.cs:line 13