Rewrote fib functions to make them insert/delete/asynchronous-walk safe.
This is implemented in a way similar to lib/slists.h, but it took some more effort to make rehashing not disturb the readers. We do it by just taking _highest_ k bits of ipa_hash as our hash value and sorting each box by whole ipa_hash(). Consult FIB_ITERATE_* macros in nest/route.h. Implemented fib_check() debugging function and also rewrote the rehashing algorithm to use better thresholds and not to waste time by rehashing forth and back.
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