CPU usage for runtime profiles with multithreading

David Rorke reports that significant amounts of time can be spent on the runtime profile with higher values of mt_dop. This can impact query performance from the client's point of view since profile serialisation is on the critical path for closing the query. Also serialising the profile for the webserver holds the ClientRequestState's lock, so can block query progress.

We should figure out how to make this more efficient.