acl-plugin: cleaner node bugfixes (VPP-675)

- use the counters in a private struct rather than node error counters
- ensure the timer for the non-idle connections is restarted
- fix the deletion of conn at the current tail the list

Change-Id: I632f63574d2ced95fb75c5e7fb588c78fb3cce1c
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>
(cherry picked from commit 097051a3bd1f63a177c0728f15375afd84a68918)