sends down UPDATE statement together with current database name.

Add MTR test for UPDATE.

Multi-UPDATE first sends SELECT to collect constrainsts for the records
to be updated. By default this SELECT uses SH that doesn't suit this.
Add if-block to guard against multi-UPDATE in create_SH().

        Renamed select handler control knob.

        Updated test suite.

      Removed unused code.

    Add a knob to disable Select handler.

        syncronize the schema list at MDB FE and CLX BE.

        Add support for RENAME TABLE.

        Fixed a double free bug in discover_table_details.

texts.

Replaced fixed size uchar arrays for buffers with memory allocations.

Fixed MDB crash in discover_table_details with CX unavailable.

Related to MDEV-21769.