Feature Request: Add API to flush all fragments for only given thread

Existing APIs for flushing fragments from the code cache (like `dr_flush_region_ex`) do so for fragments of all threads, but not for a specific thread when running with `-thread_private`. `dr_delete_fragment` does allow deleting fragments for the current thread, but doesn't help in deleting **all** fragments for that thread.

For cases where we are running with `-thread_private` and want to change instrumentation for only a given thread, it'll be useful to be able to flush fragments of the current thread only.