Create tables and schemas with lower case name only if the flag is set.
During operations, convert to lowercase in plugin. Byt the time a query gets to ExeMgr, DDLProc etc., everything must be lower case if the flag is set, and undisturbed if not.
Legacy system vars with names infinidb_* was preserved for
backward compatibility and they will be used if
columnstore_use_legacy_vars variable is set.
Remove unused structure and plugin variable.