Instead of linear search, use rhlist interface to look up the objects.
This fixes rulesets with thousands of named objects (quota, counters and
the like).

We only use a single table for this and consider the address of the
table we're doing the lookup in as a part of the key.

This reduces restore time of a sample ruleset with ~20k named counters
from 37 seconds to 0.8 seconds.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>