Introduce alter_lock to allow refusing non-online ALTER TABLE

With MDEV-16329, the majority of ALTER TABLE operations would be possible while allowing concurrent modifications to the table. Notable exceptions include ADD … AUTO_INCREMENT and DROP PRIMARY KEY without ADD PRIMARY KEY.

It could be convenient to introduce an option that allows ALTER TABLE to behave like ALTER ONLINE TABLE: refuse the operation if it would lock the table. This would be in line with the SET alter_algorithm that was introduced in MDEV-13134: allow table-rebuilding or non-instantaneous ALTER TABLE to be refused unless an explicit ALGORITHM clause is specified by the user.

The proposal is to introduce a parameter with 3 values:

-- the current situation (LOCK=DEFAULT): choose the weakest available of LOCK=SHARED and LOCK=NONE

SET alter_lock=DEFAULT;

-- disable any non-online ALTER (return an error unless explicit LOCK=DEFAULT or LOCK=SHARED is specified)

SET alter_lock=NONE;

-- disable ALTER ONLINE TABLE (lock the table; avoid allocating any logs)

SET alter_lock=SHARED;

There should be no need to implicitly add LOCK=EXCLUSIVE to each ALTER TABLE statement. That locking mode is rather useless.