cache: add hashtable cache for sets

cache: add hashtable cache for sets

This patch adds a hashtable for set lookups.

This patch also splits table->sets in two:

- Sets that reside in the cache are stored in the new
  tables->cache_set and tables->cache_set_ht.

- Set that defined via command line / ruleset file reside in
  tables->set.

Sets in the cache (already in the kernel) are not placed in the
table->sets list.

By keeping separated lists, sets defined via command line / ruleset file
can be added to cache.

Adding 10000 sets, before:

 # time nft -f x
 real    0m6,415s
 user    0m3,126s
 sys     0m3,284s

After:

 # time nft -f x
 real    0m3,949s
 user    0m0,743s
 sys     0m3,205s

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