Add, drop system versioning semantic and syntax

Here is a descriptive list how various clauses should work:

• New clause DROP PERIOD FOR SYSTEM_TIME
• ALTER TABLE DROP COLUMN sys_trx_start, DROP COLUMN sys_trx_end, DROP PERIOD FOR SYSTEM_TIME, DROP SYSTEM VERSIONING should work as ALTER TABLE DROP SYSTEM VERSIONING
• ALTER TABLE DROP COLUMN sys_trx_start, DROP COLUMN sys_trx_end, DROP PERIOD FOR SYSTEM_TIME should make fields and period invisible
• ALTER TABLE DROP COLUMN sys_trx_start, DROP COLUMN sys_trx_end does the same (the period is "dropped" implicitly when all its columns are dropped)
• DROP PERIOD FOR SYSTEM_TIME produces an error (DROP COLUMN sys_trx_start, DROP COLUMN sys_trx_end is missing)
• ALTER TABLE DROP COLUMN sys_trx_start - error (cannot drop just one column from the period)
• drop system versioning should not require invisibility of `sys_trx_*`