Unlocking the global attribute cache

Unlocking the global attribute cache

Now, when we have the unlocked slab, we can also go for a completely
lockless global attribute cache, which has been the major contention
point in BIRD 3.

This is a complicated data structure, basically a lockless hash table
with use-counted items inside, allowing concurrent insertions and
deletions.

Hash tables have to dynamically resize its base array, and the rehash
routine runs from an event if needed.

Also, due to some funny meta-race conditions, it is now possible to
end up with multiple ea_list_storage instances with the same contents,
yet it looks like it happens very rarely. (Prove us wrong, we dare you!)