[PATCH] bgp deterministic med

Hello list! This patch implements RFC 4271 MED comparison. This patch may introduce significant performance penalty (unfortunately I haven't got any real numbers at the moment). However, this comparison is absolutely necessary at least for RR setups. rte_better cost has changed from O(1) to O(n) (should not affect performance much) rte_recalculate can execute 2-3 times slower in worst case (this can consume more resources when preferred BGP neighbor goes down) Deterministic med can be made as optional global BGP config switch (which will be done in next patch version). Keeping this switch off with patched bird should not introduce much performance penalty. Please consider this patch as beta. Comments/reviews are welcome.