BUG/MINOR: checks/threads: use ha_random() and not rand()

BUG/MINOR: checks/threads: use ha_random() and not rand()

In order to honor spread_checks we currently call rand() which is not
thread safe and which must never turn its internal state to zero. This
is not thread safe, let's use ha_random() instead. This is a complement
to commimt 52bf839394 ("BUG/MEDIUM: random: implement a thread-safe and
process-safe PRNG") and may be backported with it.

(cherry picked from commit 5a6d3e797edc7ac52560cc1a5bd90a984b6b350b)
Signed-off-by: Willy Tarreau <w@1wt.eu>