Make transaction_isolation into a system variable

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.

MySQL 5.7 solved this problem by making transaction_isolation into a system variable:

https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_transaction_isolation

Same has been done to transaction_read_only option:
https://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-20.html