Add ability to only execute SELECT statements to keep slave servers warm

Im using percona-playback as per http://www.mysqlperformanceblog.com/2013/04/16/is-your-mysql-buffer-pool-warm-make-it-sweat/

To be sure the playback on the slave server is read-only, I have created a dedicated user with read-only access to the database (on top of using innodb_fake_changes).
This makes the UPDATE statements error, which as per mysql_client.cc:83, will disconnect and reconnect multiple times as it retries.
This then leaves 60000+ dangling TCP connections waiting to expire (im assuming SO_LINGER is not getting set anywhere), which then exhausts all local tcp sockets, making local connections fail.

It would be ideal to have an option to only execute select statements on the server. This would also be handy for people not running the percona server (innodb_fake_changes). It would not have to be 100% accurate as any statements that slipped through would be caught by the read-only user.