Added MXS-770 branch.

PURGE BINARY LOGS TO ‘file’; deletes all files in binlogdir and GTID
maps repo up to specified file.

mariadb10_slave_gtid=On option is needed in order to keep the list of
binlog files.

Documentation update and usage examples will be part of next commits.

Tested with mariadb10_master_gtid=On and binlog_structure=flat|tree
and with mariadb10_master_gtid=Off (implies binlog_structure=flat)

Current binlog file cannot be deleted.

Some cases:

MySQL [(none)]> purge binary logs to 'mysql-bin.00011s3';
ERROR 1198 (HY000): Cannot execute PURGE BINARY LOGS with a running slave; run STOP SLAVE first.

MySQL [(none)]> purge binary logs to 'fishy-bin.AA011s3';
ERROR 1373 (HY000): Target log not found in binlog index

MySQL [(none)]> purge binary logs to 'mysql-bin.000118';
Query OK, 0 rows affected (0.08 sec)

MaxScale/commit/113d2ad87a3352ff4a6d97a72c9d6dd3e6a711cd

In 'develop' branch