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.

util/time.c

@@ -0,0 +1,11 @@
+#define _POSIX_C_SOURCE 200809L
+#include <stdint.h>
+#include <time.h>
+
+#include "util/time.h"
+
+uint32_t get_current_time_msec(void) {
+	struct timespec now;
+	clock_gettime(CLOCK_MONOTONIC, &now);
+	return now.tv_sec * 1000 + now.tv_nsec / 1000000;
+}