lib/util/time.c: timeval_current_ofs_msec

lib/util/time.c: timeval_current_ofs_msec

Several places want "milliseconds from current time", and several were
simply doing "msec * 1000" which can (and does in one place) result in
a usec value over 1 a million.

Using a helper to do this is safer and more readable.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>