fib: fib api updates

Enhance the route add/del APIs to take a set of paths rather than just one.
Most unicast routing protocols calcualte all the available paths in one
run of the algorithm so updating all the paths at once is beneficial for the client.
two knobs control the behaviour:
  is_multipath - if set the the set of paths passed will be added to those
                 that already exist, otherwise the set will replace them.
  is_add - add or remove the set

is_add=0, is_multipath=1 and an empty set, results in deleting the route.

It is also considerably faster to add multiple paths at once, than one at a time:

vat# ip_add_del_route 1.1.1.1/32 count 100000 multipath via 10.10.10.11
100000 routes in .572240 secs, 174751.80 routes/sec
vat# ip_add_del_route 1.1.1.1/32 count 100000 multipath via 10.10.10.12
100000 routes in .528383 secs, 189256.54 routes/sec
vat# ip_add_del_route 1.1.1.1/32 count 100000 multipath via 10.10.10.13
100000 routes in .757131 secs, 132077.52 routes/sec
vat# ip_add_del_route 1.1.1.1/32 count 100000 multipath via 10.10.10.14
100000 routes in .878317 secs, 113854.12 routes/sec

vat# ip_route_add_del 1.1.1.1/32 count 100000 multipath via 10.10.10.11 via 10.10.10.12 via 10.10.10.13 via 10.10.10.14
100000 routes in .900212 secs, 111084.93 routes/sec

Change-Id: I416b93f7684745099c1adb0b33edac58c9339c1a
Signed-off-by: Neale Ranns <neale.ranns@cisco.com>
Signed-off-by: Ole Troan <ot@cisco.com>
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>