MINOR: backend: move hash_balance_factor out of chash

MINOR: backend: move hash_balance_factor out of chash

This one is a proxy option which can be inherited from defaults even
if the LB algo changes. Move it out of the lb_chash struct so that we
don't need to keep anything separate between these structs. This will
allow us to merge them into an union later. It even takes less room
now as it fills a hole and removes another one.