Changed mpm-worker to mpm-prefork

Previous configuration causes this error: DBD::mysql::dr connect failed: handle 1 is owned by thread 7f1f450a7170 not current thread 7f1f453a7400 (handles can't be shared between threads and your driver may need a CLONE method added).

Possible solution:
- http://www.redmine.org/boards/2/topics/20183?r=41109

Further readings:
- http://stackoverflow.com/questions/13883646/apache-prefork-vs-worker-mpm
- http://serverfault.com/questions/383526/how-do-i-select-which-apache-mpm-to-use/383634#383634