Make compatible gitbase with MariaDB JDBC driver

[ajnavarro]

Queries that are executed when we use MariaDB JDBC driver:

SELECT 
	@@max_allowed_packet,
	@@system_time_zone,
	@@time_zone,
	@@auto_increment_increment

set autocommit=1, 
sql_mode = concat(@@sql_mode,',STRICT_TRANS_TABLES')

We need to support:

• SET session statements
• query session variables using @@VAR_NAME or session.var_name

Caveats:

• Values like @@sql_mode or @@autocommit will be saved on session, but not used.
• Values like @@max_allowed_packet should change gitbase behavior https://dev.mysql.com/doc/refman/5.5/en/packet-too-large.html
• Values like @@time_zone should return a real value.