Rewrite statistics export

Add automatic export of metrics from slaves to the master server.  And
rewrite the statistics export plugin to use this new system.

plugins/statistics_exporter/lua/export.lua

@@ -0,0 +1,39 @@
+local statistics = {
+    "item_production_statistics",
+    "fluid_production_statistics",
+    "kill_count_statistics",
+    "entity_build_count_statistics",
+}
+
+local function export()
+    local stats = {
+        game_tick = game.tick,
+        player_count = #game.connected_players,
+        game_flow_statistics = {
+            pollution_statistics = {
+                input = game.pollution_statistics.input_counts,
+                output = game.pollution_statistics.output_counts,
+            },
+        },
+        force_flow_statistics = {}
+    }
+    for _, force in pairs(game.forces) do
+        local flow_statistics = {}
+        for _, statName in pairs(statistics) do
+            flow_statistics[statName] = {
+                input = force[statName].input_counts,
+                output = force[statName].output_counts,
+            }
+        end
+        stats.force_flow_statistics[force.name] = flow_statistics
+    end
+
+
+    rcon.print(game.table_to_json(stats))
+end
+
+return {
+    add_remote_interface = function()
+        remote.add_interface("statistics_exporter", { export = export })
+    end
+}