[IPV4]: fib_trie root node settings

The threshold for root node can be more aggressive set to get
better tree compression. The new setting mekes the root grow
from 16 to 19 bits and substansial improvemnt in Aver depth
this with the current table of 214393 prefixes

But really the dynamic resize should need more investigation
both in terms convergence and performance and maybe it should
be possible to change...

Maybe just for the brave to start with or we may have to back
this out.