MINOR: listener: replace the listener's spinlock with an rwlock

MINOR: listener: replace the listener's spinlock with an rwlock

We'll need to lock the listener a little bit more during accept() and
tests show that a spinlock is a massive performance killer, so let's
first switch to an rwlock for this lock.

This patch might have to be backported for the next patch to work, and
if so, the change is almost mechanical (look for LISTENER_LOCK), but do
not forget about the few HA_SPIN_INIT() in the file. There's no reference
to this lock outside of listener.c nor listener-t.h.