Implementation of ValidationContext inteface. Used to establish an
environment for simple type validation.
author: Elena Litani, IBM version: $Id: ValidationState.java 446719 2006-09-15 20:32:39Z mrglavas $
The same validation state can be used to validate more than one (schema)
validation roots. Entity/Namespace/Symbol are shared, but each validation
root needs its own id/idref tables. So we need this method to reset only
the two tables.