Add core-oriented drmemtrace iterator

When drmemtrace tools want to analyze a trace using the recorded schedule they may want to look at hardware threads rather than software threads. An iterator over each core would be useful for this purpose. For interactions with fast seeking #5538 the plan is to generate a schedule summary file that contains <tid, timestamp, cpuid, instr-count> tuples to make it possible to compute instruction counts on a core across multiple software threads.