Implements periodic alerts rotation

Oldest alerts are periodically overwritten by newest ones
when a certain (user-configurable) threshold has been exceeded.
Rotation is performed periodically in a housekeeping.lua

scripts/lua/modules/alert_utils.lua

@@ -123,6 +123,10 @@ function get_re_arm_alerts_hash_name()
    return "ntopng.prefs.alerts_re_arm_minutes"
 end
 
+function get_housekeeping_set_name(ifId)
+   return "ntopng.alerts.ifid_"..ifId..".make_room"
+end
+
 function get_re_arm_alerts_hash_key(ifid, ifname_or_network)
    local parts = {"ifid", tostring(ifid)}
    if ifname_or_network ~= nil then
@@ -1205,6 +1209,30 @@ end
 
 -- #################################
 
+function housekeepingAlertsMakeRoom()
+   local ifnames = interface.getIfNames()
+   for id, n in pairs(ifnames) do
+      interface.select(n)
+
+      local ifId = getInterfaceId(n)
+      local k = get_housekeeping_set_name(ifId)
+
+      local members = ntop.getMembersCache(k)
+      for _, m in pairs(members) do
+	 ntop.delMembersCache(k, m)
+	 m = m:split("|")
+
+	 local alert_entity = tonumber(m[1])
+	 local alert_entity_value = m[2]
+	 local table_name = m[3]
+
+	 interface.makeRoomAlerts(alert_entity, alert_entity_value, table_name)
+      end
+   end
+end
+
+-- #################################
+
 function drawAlertTables(num_past_alerts, num_engaged_alerts, num_flow_alerts, get_params, hide_extended_title, alt_nav_tabs)
    local alert_items = {}
    local url_params = {}