lib: vrf_bitmap_groups increased from 8 to 1024

lib: vrf_bitmap_groups increased from 8 to 1024

The number of vrf bitmap groups is increased so as to avoid consuming
too much memory. This fix is related to a fork memory that occured when
running pimd as daemon.
A check on memory consumed shows that the memory consumed goes from
33480ko to 46888ko with that change. This is less compared to if the
value of the bitmap groups is increased to 16 ( 852776ko).

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>