Implement better rebuild-delay fib algo policy.

The intent is to better handle time intervals with large amount of RIB
updates (e.g. BGP peer going up or down), while still keeping low sync
delay for the rest scenarios.

The implementation is the following: updates are bucketed into the
buckets of size 50ms. If the number of updates within a current bucket
exceeds the threshold of 500 routes/sec (e.g. 10 updates per bucket
interval), the update is delayed for another 50ms. This can be repeated
until the maximum update delay (1 sec) is reached.

All 3 variables are runtime tunables:

• net.route.algo.fib_max_sync_delay_ms: 1000
• net.route.algo.bucket_change_threshold_rate: 500
• net.route.algo.bucket_time_ms: 50

Differential Review: https://reviews.freebsd.org/D29588
MFC after: 2 weeks