echo: Fix for added delays in rule updates

echo: Fix for added delays in rule updates

The added cache update upon every command dealing with rules was a
bummer. Instead, perform the needed cache update only if echo option was
set.

Initially, I tried to perform the cache update from within
netlink_echo_callback(), but that turned into a mess since the shared
socket between cache_init() and mnl_batch_talk() would receive
unexpected new input. So instead update the cache from do_command_add(),
netlink_replace_rule_batch() and do_comand_insert() so it completes
before mnl_batch_talk() starts listening.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>