Modify TimeSlicedOutput#emit mechanizm to fit BufferedOutput#emit way

Current TimeSlicedOutput#emit emits records to buffer one by one.
It causes inconsistent state when queue has a problem.
For example, some records are stored into buffers and
raises BufferQueueLimitError. In this situation,
logs are duplicated if input plugin retries.
BufferedOutput and ObjectBufferedOutput uses another way.
These outputs format all records into data chunk first.
It is more safer than current TimeSlicedOutput way.

lib/fluent/output.rb

@@ -539,6 +539,7 @@ def configure(conf)
 
     def emit(tag, es, chain)
       @emit_count += 1
+      formatted_data = {}
       es.each {|time,record|
         tc = time / @time_slice_cache_interval
         if @before_tc == tc
@@ -548,7 +549,10 @@ def emit(tag, es, chain)
           key = @time_slicer.call(time)
           @before_key = key
         end
-        data = format(tag, time, record)
+        formatted_data[key] ||= ''
+        formatted_data[key] << format(tag, time, record)
+      }
+      formatted_data.each { |key, data|
         if @buffer.emit(key, data, chain)
           submit_flush
         end