tools/xenstore: switch write limiting to use millisecond time base

tools/xenstore: switch write limiting to use millisecond time base

There is no need to keep struct wrl_timestampt, as it serves the same
purpose as the more simple time base provided by get_now().

Move some more stuff from xenstored_domain.h into xenstored_domain.c
as it is being used nowhere else.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>