zebra: Add a timer to nexthop group deletion

Before deleting nexthop groups, that are installed,
from the system, start a timer and hold the nexthop
group for that time.

Suppose you have this scenario

a) create a static route with 1 x ecmp
      creates a nhg with 1 x ecmp
b) create a static route with 2 x ecmp
      creates a nhg with 2 x ecmp
      deletes a's nhg
c) create a static route with 3 x ecmp
      creates a nhg with 3 x ecmp
      deletes b's nhg
d) create a different route with 1 x ecmp
      creates another 1 x ecmp ( since a's ecmp was deleted )
e) create a different route with 2 x ecmp
      creates another 2 x ecmp ( since b's ecmp was deleted )

If you don't delete the nhg, start a timer, the nhg's used
in steps a and b can be reused for steps d and e.  This reduces
overhead work with zebra <-> kernel interactions and improves
the speed of the system.

So modify the code to note that an installed nexthop group should
be kept around a bit and hopefully reused.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>

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