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

input.go

@@ -24,7 +24,7 @@ type ServiceInput interface {
 	Gather(Accumulator) error
 
 	// Start starts the ServiceInput's service, whatever that may be
-	Start() error
+	Start(Accumulator) error
 
 	// Stop stops the services and closes any necessary channels and connections
 	Stop()