Make aria_pagecache_buffer_size dynamic.

Hi,

while testing the "no InnoDB Buffer Pool in core dumps" feature (details in [1]), I discovered that aria_pagecache_buffer_size is not dynamic. This is inconvenient as tuning needs restarting the server. In comparison, key_buffer_size is dynamic for a long time ([2]) and innodb_buffer_pool_size is dynamic since MariaDB 10.2 ([3]).

[1]: https://www.percona.com/community-blog/2018/06/28/nice-feature-in-mariadb-103-no-innodb-buffer-pool-in-coredumps/

[2]: https://mariadb.com/kb/en/library/myisam-system-variables/#key_buffer_size

[3]: https://mariadb.com/kb/en/library/xtradbinnodb-server-system-variables/#innodb_buffer_pool_size

Please make aria_pagecache_buffer_size dynamic.

Note: the priority of this task is set as Major. The main motivation is that the default value for aria_pagecache_buffer_size is 128 MB, which is a lot. If the default value was much smaller (8 MB or less), the priority of this task might be lowered.

Related: MDEV-16607: Consider smaller defaults for MyISAM and Aria cache sizes.

Thanks in advance,

JFG