improve route updates from kernel

[christf]

Taken from the Mailing List from a mail from @jech

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The redistribution code in babeld is very primitive:

1. the code makes a full route dump whenever something changes and computes the difference, rather than receiving incremental deltas from the kernel;
2. it uses a quadratic algorithm to compute the difference.

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[jech]

Point 2 has been adressed -- we now do things properly, in n log(n) time. Point 1 remains.