Make LockedUnboundedQueue implementation configurable

Available implementations configurations through the meson option
'locked_unbounded_queue_implementation' are:

mutex - our current LockedUnboundedQueue implementation using std::mutex

rwlock - An implementation with pthread_rwlock. The implementations tries
         to upgrade its rdlock and drops and acquires a wrlock on failure

shared_mutex - An implementation using std::shared_mutex.
         dequeue() acquires a shared lock at first, drops it and
         acquires a unique lock

boost_shared_mutex - An implementation using boost::shared_mutex.
         dequeue() acquires an upgradable lock and upgrade it
         to a unique lock if necessary