split sla from critical time

According to the history of when these headers were introduced see [here](https://github.com/Particular/NServiceBus/commit/f7110ae3e4d8506ee56599045fc3d3ff975b5688), it seems they were only introduced to support the critical time perf counter.
Now because we have the "context" to store info we do not need to store this info in headers any more. So how about we remove them?
It seems to be pointless to have 2 headers (start/stop), if we really need to expose this info in a header then one header "TotalTime" should suffice.