Add -detach_after_tracing feature to drcachesim

[derekbruening]

For a limited-duration trace, we first implemented -max_trace_size, but it's kind of useless outside of tests as it's per-thread. We later added -exit_after_tracing, but it doesn't work well with some setups where we have logic inside the app (typically using static-linked drcachesim) that needs to take some steps after DR exits. We also have -max_global_trace_refs but it has a time gap problem: #5021. For our use case what we want is a detach when a max size is reached. This requires #2644.