Flush based on buffer size rather than time

this includes:
- Add Accumulator to the Start() function of service inputs
- For message consumer plugins, use the Accumulator to constantly add
  metrics and make Gather a dummy function
- rework unit tests to match this new behavior.
- make "flush_buffer_when_full" a config option that defaults to true

closes #666

plugins/inputs/statsd/statsd.go

@@ -213,7 +213,7 @@ func (s *Statsd) Gather(acc telegraf.Accumulator) error {
 	return nil
 }
 
-func (s *Statsd) Start() error {
+func (s *Statsd) Start(_ telegraf.Accumulator) error {
 	// Make data structures
 	s.done = make(chan struct{})
 	s.in = make(chan []byte, s.AllowedPendingMessages)