handle support for mysql 8.0 tx_isolation replacement by transaction_isolation

from https://stackoverflow.com/questions/50368335/is-mariadb-java-client-2-2-3-compatible-with-mysql-8-0

Mysql 8.0.3 totally removed "tx_isolation" (https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-3.html) to rename it "transaction_isolation"
method Connection.getTransactionIsolation() rely on this variable to know current transaction isolation level.
Connector must take care of the server type/version to know to use "tx_isolation" or "transaction_isolation"