Make Releaser a sealed class

Author: maca88

src/NHibernate/Util/AsyncLock.cs

@@ -34,7 +34,7 @@ public IDisposable Lock()
 			return _releaser;
 		}
 
-		private struct Releaser : IDisposable
+		private sealed class Releaser : IDisposable
 		{
 			private readonly AsyncLock _toRelease;
 			internal Releaser(AsyncLock toRelease) { _toRelease = toRelease; }

src/NHibernate/Util/AsyncReaderWriterLock.cs

@@ -187,7 +187,7 @@ private void ReleaseWaitingReader()
 			}
 		}
 
-		private struct Releaser : IDisposable
+		private sealed class Releaser : IDisposable
 		{
 			private readonly AsyncReaderWriterLock _toRelease;
 			private readonly bool _writer;