modules/predict: cleaned up unused function, variables, trailing whitespace

The `stats` module is now also included if not exists.

```
$ luacheck modules/predict
Checking modules/predict/predict.lua              OK

Total: 0 warnings / 0 errors in 1 file
```

modules/predict/predict.lua

@@ -13,6 +13,9 @@ local predict = {
 	log = {},
 }
 
+-- Load dependent modules
+if not stats then modules.load('stats') end
+
 -- Calculate current epoch (which window fits current time)
 local function current_epoch()
 	if not predict.period or predict.period <= 1 then return nil end
@@ -27,9 +30,9 @@ local function next_event()
 end
 
 -- Resolve queued records and flush the queue
-function predict.drain(ev)
+function predict.drain()
 	local deleted = 0
-	for key, val in pairs(predict.queue) do
+	for key, _ in pairs(predict.queue) do
 		local qtype, qname = key:match('(%S*)%s(.*)')
 		worker.resolve(qname, kres.type[qtype], kres.class.IN, 'NO_CACHE')
 		predict.queue[key] = nil
@@ -48,26 +51,11 @@ function predict.drain(ev)
 	return 0
 end
 
--- Enqueue queries from set
-local function enqueue(queries)
-	local queued = 0
-	local nr_queries = #queries
-	for i = 1, nr_queries do
-		local entry = queries[i]
-		local key = string.format('%s %s', entry.type, entry.name)
-		if not predict.queue[key] then
-			predict.queue[key] = 1
-			queued = queued + 1
-		end
-	end
-	return queued	
-end
-
--- Enqueue queries from same format as predict.queue or predict.log 
+-- Enqueue queries from same format as predict.queue or predict.log
 local function enqueue_from_log(current)
 	if not current then return 0 end
-	queued = 0
-	for key, val in pairs(current) do 
+	local queued = 0
+	for key, val in pairs(current) do
 		if val and not predict.queue[key] then
 			predict.queue[key] = val
 			queued = queued + 1
@@ -79,7 +67,7 @@ end
 -- Sample current epoch, return number of sampled queries
 function predict.sample(epoch_now)
 	if not epoch_now then return 0, 0 end
-	local current = predict.log[epoch_now] or {}	
+	local current = predict.log[epoch_now] or {}
 	local queries = stats.frequent()
 	stats.clear_frequent()
 	local nr_samples = #queries
@@ -100,7 +88,7 @@ local function generate(epoch_now)
 		local current = predict.log[(epoch_now - i - 1) % predict.period + 1]
 		local past = predict.log[(epoch_now - 2*i - 1) % predict.period + 1]
 		if current and past then
-			for k, v in pairs(current) do
+			for k, _ in pairs(current) do
 				if past[k] ~= nil and not predict.queue[k] then
 					queued = queued + 1
 					predict.queue[k] = 1
@@ -111,30 +99,27 @@ local function generate(epoch_now)
 	return queued
 end
 
-function predict.process(ev)
-	if (predict.period or 0) ~= 0 and not stats then
-		error("'stats' module required")
-	end
+function predict.process()
 	-- Start a new epoch, or continue sampling
 	predict.ev_sample = nil
 	local epoch_now = current_epoch()
 	local nr_queued = 0
 
-	-- End of epoch 
+	-- End of epoch
 	if predict.epoch ~= epoch_now then
 		stats['predict.epoch'] = epoch_now
 		predict.epoch = epoch_now
-		-- enqueue records from upcoming epoch	
+		-- enqueue records from upcoming epoch
 		nr_queued = enqueue_from_log(predict.log[epoch_now])
 		-- predict next epoch
 		nr_queued = nr_queued + generate(epoch_now)
 		-- clear log for new epoch
 		predict.log[epoch_now] = {}
 	end
-	
+
 	-- Sample current epoch
 	local nr_learned = predict.sample(epoch_now)
-	
+
 	-- Dispatch predicted queries
 	if nr_queued > 0 then
 		predict.queue_len = predict.queue_len + nr_queued
@@ -151,14 +136,14 @@ function predict.process(ev)
 	collectgarbage()
 end
 
-function predict.init(module)
+function predict.init()
 	if predict.window > 0 then
 		predict.epoch = current_epoch()
 		predict.ev_sample = event.after(next_event(), predict.process)
 	end
 end
 
-function predict.deinit(module)
+function predict.deinit()
 	if predict.ev_sample then event.cancel(predict.ev_sample) end
 	if predict.ev_drain then event.cancel(predict.ev_drain) end
 	predict.ev_sample = nil
@@ -182,7 +167,7 @@ end
 predict.layer = {
 	-- Prefetch all expiring (sub-)queries immediately after the request finishes.
 	-- Doing that immediately is simplest and avoids creating (new) large bursts of activity.
-	finish = function (state, req)
+	finish = function (_, req)
 		req = kres.request_t(req)
 		local qrys = req.rplan.resolved
 		for i = 0, (tonumber(qrys.len) - 1) do -- size_t doesn't work for some reason