Close connection on update_event error for new cmd

Issue 261. If an update_event fails for case conn_new_cmd but no
more events happen on this socket, conn_closing will never be
processed.

Not sure if it's possible for this to happen, but it could
theoretically leak refcounts.
Do not do internal item_realloc() every time

refcount == 2 is "linked and I own it"
code was doing a replace if refcount != 1 (which would mean unlinked and I own
it, in this case. If that code ever actually misfired some weird stuff might
happen).