Implement a per-output fixed size metric buffer

Also moved some objects out of config.go and put them in their own
package, internal/models

fixes #568
closes #285

internal/models/running_input.go

@@ -0,0 +1,24 @@
+package models
+
+import (
+	"time"
+
+	"github.com/influxdata/telegraf/plugins/inputs"
+)
+
+type RunningInput struct {
+	Name   string
+	Input  inputs.Input
+	Config *InputConfig
+}
+
+// InputConfig containing a name, interval, and filter
+type InputConfig struct {
+	Name              string
+	NameOverride      string
+	MeasurementPrefix string
+	MeasurementSuffix string
+	Tags              map[string]string
+	Filter            Filter
+	Interval          time.Duration
+}