Refactor handling of MinMax functionality into RunningAggregator

allows for easier addition of a sliding window at a later time.

Also makes `period` be a generic argument for all aggregator plugins.

aggregator.go

@@ -1,16 +1,22 @@
 package telegraf
 
+// Aggregator is an interface for implementing an Aggregator plugin.
+// the RunningAggregator wraps this interface and guarantees that
+// Add, Push, and Reset can not be called concurrently, so locking is not
+// required when implementing an Aggregator plugin.
 type Aggregator interface {
-	// SampleConfig returns the default configuration of the Input
+	// SampleConfig returns the default configuration of the Input.
 	SampleConfig() string
 
-	// Description returns a one-sentence description on the Input
+	// Description returns a one-sentence description on the Input.
 	Description() string
 
-	// Apply the metric to the aggregator
-	Apply(in Metric)
+	// Add the metric to the aggregator.
+	Add(in Metric)
 
-	// Start starts the service filter with the given accumulator
-	Start(acc Accumulator) error
-	Stop()
+	// Push pushes the current aggregates to the accumulator.
+	Push(acc Accumulator)
+
+	// Reset resets the aggregators caches and aggregates.
+	Reset()
 }