Mtrie optimisations

Mtrie optimisations

1 - make the default route non-special, i.e. like any other less specific route. Consequently, all buckets have a valid valid index of either a leaf or a ply. Checks for special indeices in the data-path can thus be removed.
2 - since all leaves are now 'real' i.e. they represent a real load-balance object, to tell if a ply slot is 'empty' requeirs chekcing that the prefix length of the leaf occupying the slot is slot than the minium value for that ply.
3 - when removing a leaf find the cover first, then recurse down the ply and replace the old leaf with the cover. This saves us a ply walk.

Change-Id: Idd523019e8bb1b6ef527b1f5279a5e24bcf18332
Signed-off-by: Neale Ranns <nranns@cisco.com>