This worklog prohibits access to the internal tables
of the transactional data dictionary (DD). The worklog
implements the following main changes:

1. Remove unused data structures and plugin usage. In
   particular, please note that the 'system_database()'
   handlerton function has been removed.

2. Prohibit access for usage of DD tables from:

   - SQL statements.
   - Foreign key constraints.
   - Stored programs.
   - Replication (also cross version).

   Access is accepted if the thread executing the
   statement is a DD system thread. For external threads,
   i.e., representing an external client, access is accepted
   if the table is a DDSE table (e.g. the 'innodb_*_stats'
   tables), and the statement is DML.

3. Add a debug flag 'skip_dd_table_access_check' to allow
   DD table access in debug builds. Rewrite tests to set
   this flag when needing access to the DD tables. Please
   note that great care must be taken when this flag is
   used. If using DML to update the contents of the DD
   tables directly, without any corresponding updates
   in the storage engine or DD cache, invariants that
   the server expectes to hold will be broken, and a
   malfunctioning server is to be expected. The same
   goes for DDL; with a change in the definitions of the
   DD tables, it is to be expected that the server will
   not work as intended.

4. Rewrite tests that currently access DD tables to set
   the debug flag above. Separate out specific parts of
   tests to only be run on debug builds, if the test
   previously could run on non-debug builds as well.

5. Take into account that the access check mechanism
   may be invoked from the unit tests; check that the
   Dictionary instance exists before calling it.

TODO: Dependencies on other worklogs:

1. Remove exception regarding queries against the
   st_spatial_refrerence_systems table (WL#9059).
2. Prohibit access to altering or dropping the system
   tablespace (WL#7141).