netfilter: conntrack: merge ecache and timestamp sysctl tables with main one

Similar to previous change, this time for eache and timestamp.
Unlike helper and acct, these can be disabled at build time, so they
need ifdef guards.

Next patch will remove a few (now obsolete) functions.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>