Add a get_gdb_thread_id function to mythbaseutil.{h,cpp}
Use gdb thread id in logging
Add rename_thread() to make thread pool more efficient.

If we call deregister_thread() + register_thread() for every runnable
we run then it means we call get_gdb_thread_id() for every runnable;
this instead just renames the pre-existing ThreadInfo for the new
runnable.