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/
MySQL: determine slave-ness before stopping

This makes it easier to perform later logic (to use SLAVE STATUS or
MASTER STATUS).
Mysql: use Exec_Master_Log_Pos

When we restore this snapshot, we tell mysql "This is the position we
were at" and that corresponds with "Exec_Master_Log_Pos" (the pos of
the last executed SQL command). "Read_Master_Log_Pos" is the position of
the last entry written to the relay log which we dont' care about since
we're not saving the relay log and not setting it's position on restore.