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/rtt/web_gui/get_rtt_hosts.lua

@@ -0,0 +1,143 @@
+--
+-- (C) 2019 - ntop.org
+--
+
+local dirs = ntop.getDirs()
+package.path = dirs.installdir .. "/scripts/lua/modules/?.lua;" .. package.path
+
+require "lua_utils"
+local format_utils = require("format_utils")
+local json = require("dkjson")
+local ts_utils = require("ts_utils")
+local rtt_utils = require("rtt_utils")
+local plugins_utils = require("plugins_utils")
+
+sendHTTPContentTypeHeader('application/json')
+
+-- ################################################
+
+local currentPage   = _GET["currentPage"]
+local perPage       = _GET["perPage"]
+local sortColumn    = _GET["sortColumn"]
+local sortOrder     = _GET["sortOrder"]
+local cont_filter_s = _GET["custom_hosts"]
+local rtt_host      = _GET["rtt_host"]
+
+local sortPrefs = "rtt_hosts"
+
+-- ################################################
+
+if isEmptyString(sortColumn) or sortColumn == "column_" then
+   sortColumn = getDefaultTableSort(sortPrefs)
+else
+   if((sortColumn ~= "column_")
+    and (sortColumn ~= "")) then
+      tablePreferences("sort_"..sortPrefs, sortColumn)
+   end
+end
+
+if isEmptyString(_GET["sortColumn"]) then
+  sortOrder = getDefaultTableSortOrder(sortPrefs, true)
+end
+
+if((_GET["sortColumn"] ~= "column_")
+  and (_GET["sortColumn"] ~= "")) then
+    tablePreferences("sort_order_"..sortPrefs, sortOrder, true)
+end
+
+if(currentPage == nil) then
+   currentPage = 1
+else
+   currentPage = tonumber(currentPage)
+end
+
+if(perPage == nil) then
+   perPage = getDefaultTableSize()
+else
+   perPage = tonumber(perPage)
+   tablePreferences("rows_number", perPage)
+end
+
+local sOrder = ternary(sortOrder == "desc", rev_insensitive, asc_insensitive)
+local to_skip = (currentPage-1) * perPage
+
+-- ################################################
+
+local rtt_hosts = rtt_utils.getHosts()
+local totalRows = 0
+local sort_to_key = {}
+
+if rtt_host then
+   if rtt_hosts[rtt_host] then
+      sort_to_key[rtt_host] = rtt_hosts[rtt_host]["host"]
+      totalRows = 1
+   end
+else
+   for key, config in pairs(rtt_hosts) do
+      sort_to_key[key] = config.host
+
+      totalRows = totalRows + 1
+   end
+end
+
+ --################################################
+
+local res = {}
+local i = 0
+
+for key in pairsByValues(sort_to_key, sOrder) do
+  if i >= to_skip + perPage then
+    break
+  end
+
+  if (i >= to_skip) then
+    local rtt_host = rtt_hosts[key]
+    local chart = ""
+
+    if ts_utils.exists("monitored_host:rtt", {ifid=getSystemInterfaceId(), host=key}) then
+      chart = '<a href="'.. plugins_utils.getUrl('rtt_stats.lua') .. '?rtt_host='.. key ..'&page=historical"><i class="fa fa-chart-area fa-lg"></i></a>'
+    end
+
+    local column_last_ip = ""
+    local column_last_update = ""
+    local column_last_rtt = ""
+    local last_update = rtt_utils.getLastRttUpdate(key)
+
+    if(last_update ~= nil) then
+      local tdiff = os.time() - last_update.when
+
+      if(tdiff <= 600) then
+        column_last_update  = secondsToTime(tdiff).. " " ..i18n("details.ago")
+      else
+        column_last_update = format_utils.formatPastEpochShort(last_update.when)
+      end
+
+      column_last_rtt = last_update.value .. " ms"
+      column_last_ip = last_update.ip
+    end
+ 
+    res[#res + 1] = {
+      column_key = key,
+      column_host = rtt_host.host,
+      column_chart = chart,
+      column_iptype = rtt_host.iptype,
+      column_max_rrt = rtt_host.max_rtt,
+      column_last_rrt = column_last_rtt,
+      column_last_update = column_last_update,
+      column_last_ip = column_last_ip,
+    }
+  end
+
+  i = i + 1
+end
+
+-- ################################################
+
+local result = {}
+result["perPage"] = perPage
+result["currentPage"] = currentPage
+result["totalRows"] = totalRows
+result["data"] = res
+result["sort"] = {{sortColumn, sortOrder}}
+
+print(json.encode(result))