0xFFFFFFFF milliseconds is 4,294,967,295 ms so about 50 days.
A little bit too close for comfort.
Use int64_t instead of uint64_t to avoid C's implicit conversion
footguns in computations.
This commit makes `get_current_time_msec` correctly return milliseconds
as opposed to microseconds. It also considers the value of `tv_sec`, so
we don't lose occasionally go back in time by one second. Finally, the
function is moved into `util/time.cc` so that it can be reused elsewhere
without having to consider these pitfalls.
