Periodic discovery.

Prior implementation only sends Low Level Discovery JSON only when
discovered items has changed. However, this was a problematic behavior
because not always Zabbix acknowledges the LLD JSON (template was
not attached to the host at the time, or config-syncer has not
collected the LLD item yet). Therefore, use build_discovery_items() method
to send LLD JSONs periodically, irrespective of the discovered items.