threads: call `vlc_cond_t` signal/broadcast while locked

MR following the discussion here: !4883 (comment 425357)

One less vlc_mutex_t lock/unlock for each vlc_cond_t wait/signal/broadcast call.

Most VLC codebase was already calling vlc_cond_t signal/broadcast with a lock held, except in the 10 first commits of this MR.