MINOR: time: replace calls to tv_ms_elapsed() with a linear subtract

MINOR: time: replace calls to tv_ms_elapsed() with a linear subtract

Instead of operating on {sec, usec} now we convert both operands to
ns then subtract them and convert to ms. This is a first step towards
dropping timeval from these timestamps.

Interestingly, tv_ms_elapsed() and tv_ms_remain() are no longer used at
all and could be removed.