- Moving some of Database_qualidied_name methods into a new class
  Identifier_chain2.

- Changing the data type of the following variables from
  Database_qualified_name to Identifier_chain2:

  * q_pkg_proc in LEX::call_statement_start()
  * q_pkg_func in LEX::make_item_func_call_generic()

Rationale:

The data type of Database_qualified_name::m_db will be changed
to Lex_ident_db soon. So Database_qualified_name won't be able
to store the `pkg.routine` part of `db.pkg.routine` any more,
because `pkg` must not depend on lower-case-table-names.