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

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".

(cherry picked from commit 8cc586c73fefd96f4be1f7820e38a1263f6252ca)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 049602fff734664afbf75b5b28b1d9a028ed9f04)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 84c81577e52b7376e2c03cfd552d3023fa4349ec)
Signed-off-by: Willy Tarreau <w@1wt.eu>