It's now possible to pause iteration through hash. This requires
struct hash_iterator to be allocated somewhere handy.
The iteration itself is surrounded by HASH_WALK_ITER and
HASH_WALK_ITER_END. Call HASH_WALK_ITER_PUT to ask for pausing; it may
still do some more iterations until it comes to a suitable pausing
point. The iterator must be initalized to an empty structure. No cleanup
is needed if iteration is abandoned inbetween.
Prefix and bucket tables are initialized when entering established state
but not explicitly freed when leaving it (that is handled by protocol
restart). With graceful restart, BGP may enter and leave established
state multiple times without hard protocol restart causing memory leak.
