Starting/stopping cost centre counters

We have startProfTimer/stopProfTimer for controlling the timer for ticks in cost centre profiling, but this doesn't affect the entries and allocation counts of the cost centre stacks. It would be nice if we could freeze all profiling counters.

I've made an implementation of this functionality in !12440, which I found helpful for analyzing the behaviour of a long-running program while performing specific tasks or processing specific data.