src: remove cache lookups after the evaluation phase

src: remove cache lookups after the evaluation phase

This patch adds a new field to the cmd structure for elements to store a
reference to the set. This saves an extra lookup in the netlink bytecode
generation step.

This patch also allows to incrementally update during the evaluation
phase according to the command actions, which is required by the follow
up ("evaluate: remove table from cache on delete table") bugfix patch.

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