Refactor commandHandler to use commonlib implementation

[stvhwrd]

https://github.com/stvhwrd/SENG-468-HTTP-Server/blob/666cd64f986df75d0320420d175af8f18cd8637d/main.go#L87

"This should be handled by commonlib. The workload generator already composes the messages correctly, so all you need to do is have a switch to handle the message type and go from there. See transaction server for reference."