Inject kernel context switch sequences in the drmemtrace scheduler

Just like we're adding canned kernel syscall sequences in #6495, we'd like to insert some roughly-representative kernel context switch sequence as well. Like the syscalls this will probably be fed by a file, or files if we want to support different types of switches, which the scheduler will insert at switch points. These injected records presumably will count toward output ordinals, but not toward preempt quanta, and won't have any corresponding input.