bgpd: Further refine hash lookup to store hash value

bgpd: Further refine hash lookup to store hash value

Further refine the previous commit to store the hash value in
both the `struct community_list` as well as the `struct rmap_community`
structures.  This allows us to know a priori what our hash value
is.  This change cuts another couple of seconds of convergence
off to ~55 seconds and further reduces cpu load of bgp:

   16      40061.706    433732       92    330102      129   1242965 RWTEX TOTAL

Down from ~43 seconds previously.

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