Slave recovery doesn't work if session commands have been executed

If a session command has been executed, slave servers will never be recovered.

Steps to reproduce

1. Start MaxScale with 1 master and 2 slaves
2. Connect to MaxScale with Readwritesplit
3. Execute SET @a=1
4. Block first slave
5. Wait until monitor detects it
6. Unblock first slave and block the second slave
7. First slave is not recovered