Mysql: add option for using "STOP SLAVE"

Instead of "FLUSH TABLES WITH READ LOCK"

mysql has an issue with read locks and it's very easy to effectively
lock up your whole DB with a SELECT. If a mysql slave is being
snapshotted, "STOP SLAVE" is enough to ensure no more writes happen and
is guaranteed not to lock the DB.

See: http://www.mysqlperformanceblog.com/2012/03/23/how-flush-tables-with-read-lock-works-with-innodb-tables/