OPENDJ-3089 Update SchemaBackend to use SchemaBuilder when replacing an entry

In SchemaBackend class
- Update replaceEntry() and all dependent methods, including methods addX()
/ removeX() for schema elements
- In addX() methods, rely on initial schema instead of an up-to-date schema
in order to avoid generating a full schema each time
- In removeX() methods, rely on up-to-date schema because it could wrgonly
throw an exception if based on initial schema

Replace calls to server's Schema static methods by calls to SchemaHandler
static methods