Implement ntopng plugins

Plugins are a convenient way to group together related lua scripts.
Their primary use case is to group user scripts and their alert/status
definition.
The builtin ntopng user scripts and definitions are now
packed into plugins directories. In future, we will support loading of
user created plugins.
Plugins are loaded at startup into some runtime directories and then
used. Other changes provided by this commit include:

- Add sample flow logger plugin
- Initial support for system user scripts
- Rename edge to threshold
- Migrate system probes to user scripts/plugins
- Migrate scripts to more explicit alerts_api.checkThresholdAlert api

scripts/plugins/request_reply_ratio/user_scripts/host/request_reply_ratio.lua

@@ -0,0 +1,79 @@
+--
+-- (C) 2019 - ntop.org
+--
+
+local alerts_api = require("alerts_api")
+local user_scripts = require("user_scripts")
+
+local script
+
+-- #################################################################
+
+local function request_reply_ratio(params)
+  local dns_info = host.getDNSInfo()
+  local http_info = host.getHTTPInfo()
+
+  -- {requests, replies}
+  local to_check = {}
+
+  if(dns_info["dns"] ~= nil) then
+    to_check["dns_sent"] = {dns_info["dns"]["sent"]["num_queries"], (dns_info["dns"]["rcvd"]["num_replies_ok"] + dns_info["dns"]["rcvd"]["num_replies_error"])}
+    to_check["dns_rcvd"] = {dns_info["dns"]["rcvd"]["num_queries"], (dns_info["dns"]["sent"]["num_replies_ok"] + dns_info["dns"]["sent"]["num_replies_error"])}
+  end
+
+  if(http_info["http"] ~= nil) then
+    to_check["http_sent"] = {http_info["http"]["sender"]["query"]["total"], http_info["http"]["receiver"]["response"]["total"]}
+    to_check["http_rcvd"] = {http_info["http"]["receiver"]["query"]["total"], http_info["http"]["sender"]["response"]["total"]}
+  end
+
+  for key, values in pairs(to_check) do
+    local to_check_key = script.key .. "__" .. key
+
+    -- true to avoid generating an alert due to a value just restored from redis
+    local skip_first = true
+    local requests = alerts_api.host_delta_val(to_check_key .. "_requests", params.granularity, values[1], skip_first)
+    local replies = alerts_api.host_delta_val(to_check_key .. "_replies", params.granularity, values[2], skip_first)
+    local ratio = (replies * 100) / (requests+1)
+    local req_repl_type = alerts_api.requestReplyRatioType(key, requests, replies, params.granularity)
+
+    -- 10: some meaningful value
+    if((requests + replies > 10) and (ratio < tonumber(params.alert_config.threshold))) then
+      alerts_api.trigger(params.alert_entity, req_repl_type, nil, params.cur_alerts)
+    else
+      alerts_api.release(params.alert_entity, req_repl_type, nil, params.cur_alerts)
+    end
+  end
+end
+
+-- #################################################################
+
+script = {
+  local_only = true,
+  nedge_exclude = true,
+  default_enabled = true,
+
+  hooks = {
+    ["5mins"] = request_reply_ratio
+  },
+
+  default_value = {
+    -- "< 50%"
+    operator = "lt",
+    threshold = 50,
+  },
+
+  gui = {
+    i18n_title = "entity_thresholds.request_reply_ratio_title",
+    i18n_description = "entity_thresholds.request_reply_ratio_description",
+    i18n_field_unit = user_scripts.field_units.percentage,
+    input_builder = user_scripts.threshold_cross_input_builder,
+    post_handler = user_scripts.threshold_cross_post_handler,
+    field_max = 100,
+    field_min = 1,
+    field_operator = "lt";
+  }
+}
+
+-- #################################################################
+
+return script