The atomic variable used in ServiceExecutorFixedFixture::FlattenRecursiveScheduledTasks (see here) to track the recursion depth should be replaced by a thread-local integer, making it safe for multiple executor threads to run tasks without concurrently bumping up the recursion depth.

Having a thread-local variable to track recursion depth is necessary since another executor thread (i.e., exec2) may start executing a task immediately after it's been scheduled by another thread (i.e., exec1), making it appear as the thread that scheduled the task (exec1) is executing it recursively.