[MINOR] add curr_sec_ms and curr_sec_ms_scaled for current second.

[MINOR] add curr_sec_ms and curr_sec_ms_scaled for current second.

Several algorithms will need to know the millisecond value within
the current second. Instead of doing a divide every time it is needed,
it's better to compute it when it changes, which is when now and now_ms
are recomputed.

curr_sec_ms_scaled is the same multiplied by 2^32/1000, which will be
useful to compute some ratios based on the position within last second.