MINOR: backend: replace the lbprm lock with an rwlock

MINOR: backend: replace the lbprm lock with an rwlock

It was previously a spinlock, and it happens that a number of LB algos
only lock it for lookups, without performing any modification. Let's
first turn it to an rwlock and w-lock it everywhere. This is strictly
identical.

It was carefully checked that every HA_SPIN_LOCK() was turned to
HA_RWLOCK_WRLOCK() and that HA_SPIN_UNLOCK() was turned to
HA_RWLOCK_WRUNLOCK() on this lock. _INIT and _DESTROY were updated too.