Make SessionFactoryImpl.imports a ReadOnlyDictionary to line up with how collectionMetadata is built and improve determistic behavior under accidental multithreading

Author: mjkent

src/NHibernate/Impl/SessionFactoryImpl.cs

@@ -141,7 +141,7 @@ public void HandleEntityNotFound(string entityName, string propertyName, object
 		private readonly Dictionary<string, IIdentifierGenerator> identifierGenerators;
 
 		[NonSerialized]
-		private readonly IDictionary<string, string> imports;
+		private readonly IReadOnlyDictionary<string, string> imports;
 
 		[NonSerialized]
 		private readonly IInterceptor interceptor;