Make Clear/ClearAsync virtual

Author: hazzik

src/NHibernate/Async/Cache/UpdateTimestampsCache.cs

@@ -25,7 +25,7 @@ public partial class UpdateTimestampsCache
 		private readonly NHibernate.Util.AsyncLock _invalidate = new NHibernate.Util.AsyncLock();
 		private readonly NHibernate.Util.AsyncLock _isUpToDate = new NHibernate.Util.AsyncLock();
 
-		public Task ClearAsync(CancellationToken cancellationToken)
+		public virtual Task ClearAsync(CancellationToken cancellationToken)
 		{
 			if (cancellationToken.IsCancellationRequested)
 			{

src/NHibernate/Cache/UpdateTimestampsCache.cs

@@ -21,7 +21,7 @@ public partial class UpdateTimestampsCache
 
 		private readonly string regionName = typeof(UpdateTimestampsCache).Name;
 
-		public void Clear()
+		public virtual void Clear()
 		{
 			updateTimestamps.Clear();
 		}