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/github_webhooks/github_webhooks.go

@@ -61,7 +61,7 @@ func (gh *GithubWebhooks) Listen() {
 	}
 }
 
-func (gh *GithubWebhooks) Start() error {
+func (gh *GithubWebhooks) Start(_ telegraf.Accumulator) error {
 	go gh.Listen()
 	log.Printf("Started the github_webhooks service on %s\n", gh.ServiceAddress)
 	return nil