WL#4761: Streamline OPEN and CLOSE of tables

The intention of this task is to simplify the external interface
to open and close tables, to make it clearer what each method
can do and can't do and ensure that it there is no code duplication
in this area.

This is preparatory work to get better control over metadata in the
MySQL Server as well as to prepare for improved scalability by
changing the way LOCK_open is handled and later also LOCK_mdl.