util: fix and move get_current_time_msec into a util file

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.

include/util/time.h

@@ -0,0 +1,9 @@
+#ifndef UTIL_TIME_H
+#define UTIL_TIME_H
+
+/**
+ * Get the current time, in milliseconds.
+ */
+uint32_t get_current_time_msec(void);
+
+#endif