stats: extract constructor StatsCluster

This patch adds a separate constructor function for StatsCluster as
stats_cluster_new() and invokes that from stats-registry instead
of open coding it.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
stats: remove new argument from stats registration

This argument was practically unused, so get rid off that.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>