In MariaDB, we have a confusing problem where:
* The transaction_isolation option can be set in a configuration file, but it cannot be set dynamically.
* The tx_isolation system variable can be set dynamically, but it cannot be set in a configuration file.

Therefore, we have two different names for the same thing in different contexts. This is needlessly confusing, and it complicates the documentation. The same thing applys for transaction_read_only.

MySQL 5.7 solved this problem by making them into system variables. https://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-20.html

This commit takes a similar approach by adding new system variables and marking the original ones as deprecated. This commit also resolves some legacy problems related to SET STATEMENT and transaction_isolation.

Commit a923d6f49c1ad6fd3f4d6ec02e444c26e4d1dfa8 disabled numeric setting
of character_set_* variables with non-default values:

  MariaDB [(none)]> set character_set_client=224;
  ERROR 1115 (42000): Unknown character set: '224'

However the corresponding binlog functionality still write numeric
values for log event, and this will break binlog replay if the value is
not default. Now make the server use 'String' type for
'character_set_client' when generating binlog events

Before:

  /*!\C utf8mb4 *//*!*/;
  SET @@session.character_set_client=224,@@session.collation_connection=224,@@session.collation_server=33/*!*/;

After:

  /*!\C utf8mb4 *//*!*/;
  SET @@session.character_set_client=utf8mb4,@@session.collation_connection=33,@@session.collation_server=8/*!*/;

Note: prior to the previous commit, setting with '224' or '45' or
'utf8mb4' have the same effect, as they all set the parameter to
'utf8mb4'.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.