FIB memory leaks (VPP-578)

1) vec_free the fe_srcs of a fib_entry_t when the fib_entry_t is itself reed
2) in the load-balance fixup if a drop path is required add this to a new vector of next-hops 'fixed_nhs'. This vector is managed by the load-balance function. The caller continues to manage its own set. The function is now const implying that the caller is safe to assume the next-hops do not change.

Change-Id: I0f29203ee16b9a270f40edf237488fa99ba65320
Signed-off-by: Neale Ranns <nranns@cisco.com>
Signed-off-by: Neale Ranns <neale.ranns@cisco.com>