Fix: urcu-bp interaction with threads vs constructors/destructors

Add a reference counter for threads using urcu-bp, thus ensuring that
even if the urcu destructor is executed before each thread using RCU
read-side critical sections exit, those threads will not see a corrupted
thread list.

Also, don't use URCU_TLS() within urcu_bp_thread_exit_notifier(). It
appears that this is racy (although this was probably due to the issue
fixed by reference counting). Anyway, play safe, and pass the rcu_key
received as parameter instead.

Those issues only reproduce when threads are still active when the
urcu-bp destructor is called.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>