util/time: use int64_t return value for get_current_time_msec()

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.

include/util/time.h

@@ -6,7 +6,7 @@
 /**
  * Get the current time, in milliseconds.
  */
-uint32_t get_current_time_msec(void);
+int64_t get_current_time_msec(void);
 
 /**
  * Convert a timespec to milliseconds.