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/influxdb_monitor/ts_schemas/5mins.lua

@@ -0,0 +1,94 @@
+--
+-- (C) 2019 - ntop.org
+--
+
+local ts_utils = require "ts_utils_core"
+local schema
+
+-- ##############################################
+
+local function get_memory_size_query(influxdb, schema, tstart, tend, time_step)
+   --[[
+      See comments in function driver:getMemoryUsage() to understand
+      why it is necessary to subtract the HeapReleased from Sys.
+   --]]
+   local q = 'SELECT MEAN(Sys) - MEAN(HeapReleased) as mem_bytes' ..
+      ' FROM "_internal".."runtime"' ..
+      " WHERE time >= " .. tstart .. "000000000 AND time <= " .. tend .. "000000000" ..
+      " GROUP BY TIME(".. time_step .."s)"
+
+   return(q)
+end
+
+local function get_write_success_query(influxdb, schema, tstart, tend, time_step)
+   local q = 'SELECT SUM(writePointsOk) as points' ..
+      ' FROM (SELECT '..
+      ' (DERIVATIVE(MEAN(writePointsOk)) / '.. time_step ..') as writePointsOk' ..
+      ' FROM "monitor"."shard" WHERE "database"=\''.. influxdb.db ..'\'' ..
+      " AND time >= " .. tstart .. "000000000 AND time <= " .. tend .. "000000000" ..
+      " GROUP BY id)" ..
+      " GROUP BY TIME(".. time_step .."s)"
+
+   return(q)
+end
+
+-- ##############################################
+
+schema = ts_utils.newSchema("influxdb:storage_size", {
+      metrics_type = ts_utils.metrics.gauge,
+      is_system_schema = true,
+      step = 300,
+})
+schema:addTag("ifid")
+schema:addMetric("disk_bytes")
+
+schema = ts_utils.newSchema("influxdb:exported_points", {
+      metrics_type = ts_utils.metrics.counter,
+      is_system_schema = true,
+      step = 300,
+})
+schema:addTag("ifid")
+schema:addMetric("points")
+
+schema = ts_utils.newSchema("influxdb:dropped_points", {
+      metrics_type = ts_utils.metrics.counter,
+      is_system_schema = true,
+      step = 300,
+})
+schema:addTag("ifid")
+schema:addMetric("points")
+
+schema = ts_utils.newSchema("influxdb:exports", {
+      metrics_type = ts_utils.metrics.counter,
+      is_system_schema = true,
+      step = 300,
+})
+schema:addTag("ifid")
+schema:addMetric("num_exports")
+
+schema = ts_utils.newSchema("influxdb:rtt", {
+      metrics_type = ts_utils.metrics.gauge,
+      is_system_schema = true,
+      step = 300,
+})
+schema:addTag("ifid")
+schema:addMetric("millis_rtt")
+
+-- The following metrics are built-in into influxdb
+schema = ts_utils.newSchema("influxdb:memory_size", {
+      influx_internal_query = get_memory_size_query,
+      is_system_schema = true,
+      metrics_type = ts_utils.metrics.gauge,
+      step = 10,
+})
+schema:addTag("ifid")
+schema:addMetric("mem_bytes")
+
+schema = ts_utils.newSchema("influxdb:write_successes", {
+      influx_internal_query = get_write_success_query,
+      is_system_schema = true,
+      metrics_type = ts_utils.metrics.counter,
+      step = 10,
+})
+schema:addTag("ifid")
+schema:addMetric("points")