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 Dictionary<string, string> imports;
+		private readonly IDictionary<string, string> imports;
 
 		[NonSerialized]
 		private readonly IInterceptor interceptor;
@@ -357,7 +357,7 @@ public SessionFactoryImpl(Configuration cfg, IMapping mapping, Settings settings
 			sqlResultSetMappings = new Dictionary<string, ResultSetMappingDefinition>(cfg.SqlResultSetMappings);
 			#endregion
 
-			imports = new Dictionary<string, string>(cfg.Imports);
+			imports = new ReadOnlyDictionary<string, string>(cfg.Imports);
 
 			#region after *all* persisters and named queries are registered
 			foreach (IEntityPersister persister in entityPersisters.Values)