Invoker

We record that the given id has been invoked by appending its id to the coverage data file.

This will happen concurrently on as many threads as the application is using, so we use one file per thread, named for the thread id.

This method is not thread-safe if the threads are in different JVMs, because the thread IDs may collide. You may not use scoverage on multiple processes in parallel without risking corruption of the measurement file.