BGP: Export uses common attribute cache

BGP: Export uses common attribute cache

There is no real need for storing bucket attributes locally and we may
save some memory by caching the attributes in one central place.

This was a contention problem though, so this commit got reverted,
but now the attribute cache is actually lockless (yay!) and we can
try it again.

If this becomes a contention problem, we should ... no, please not.
Not again! Please!

(Dug out from the history and updated by Katerina Kubecova.)