[fib_algo][dxr] Improve incremental updating strategy
Tracking the number of unused holes in the trie and the range table
was a bad metric based on which full trie and / or range rebuilds
were triggered, which would happen in vain by far too frequently,
particularly with live BGP feeds.
Instead, track the total unused space inside the trie and range table
structures, and trigger rebuilds if the percentage of unused space
exceeds a sysctl-tunable threshold.
MFC after: 3 days