src: add chain hashtable cache

src: add chain hashtable cache

This significantly improves ruleset listing time with large rulesets
(~50k rules) with _lots_ of non-base chains.

 # time nft list ruleset &> /dev/null

Before this patch:

real    0m11,172s
user    0m6,810s
sys     0m4,220s

After this patch:

real    0m4,747s
user    0m0,802s
sys     0m3,912s

This patch also removes list_bindings from netlink_ctx since there is no
need to keep a temporary list of chains anymore.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>