BUG/MEDIUM: freq_ctr/threads: use the global_now_ms variable

In commit a1ecbca0a ("BUG/MINOR: freq_ctr/threads: make use of the last
updated global time"), for period-based counters, the millisecond part
of the global_now variable was used as the date for the new period. But
it's wrong, it only works with sub-second periods as it wraps every
second, and for other periods the counters never rotate anymore.

Let's make use of the newly introduced global_now_ms variable instead,
which contains the global monotonic time expressed in milliseconds.

This patch needs to be backported wherever the patch above is backported.
It depends on previous commit "MINOR: time: also provide a global,
monotonic global_now_ms timer".
diff --git a/src/freq_ctr.c b/src/freq_ctr.c
index 8fd0c90..42fbc1c 100644
--- a/src/freq_ctr.c
+++ b/src/freq_ctr.c
@@ -206,7 +206,7 @@
 			break;
 	};
 
-	remain = curr_tick + period - (uint32_t)global_now / 1000;
+	remain = curr_tick + period - global_now_ms;
 	if (unlikely((int)remain < 0)) {
 		/* We're past the first period, check if we can still report a
 		 * part of last period or if we're too far away.
@@ -253,7 +253,7 @@
 			break;
 	};
 
-	remain = curr_tick + period - (uint32_t)global_now / 1000;
+	remain = curr_tick + period - global_now_ms;
 	if (likely((int)remain < 0)) {
 		/* We're past the first period, check if we can still report a
 		 * part of last period or if we're too far away.