Improve the InfluxDB through-put performance

This changes the current use of the InfluxDB client to instead use a
baked-in client that uses the fasthttp library.

This allows for significantly smaller allocations, the re-use of http
body buffers, and the re-use of the actual bytes of the line-protocol
metric representations.

metric.go

@@ -19,8 +19,15 @@ const (
 )
 
 type Metric interface {
+	// Serialize serializes the metric into a line-protocol byte buffer,
+	// including a newline at the end.
 	Serialize() []byte
-	String() string // convenience function for string(Serialize())
+	// same as Serialize, but avoids an allocation.
+	// returns number of bytes copied into dst.
+	SerializeTo(dst []byte) int
+	// String is the same as Serialize, but returns a string.
+	String() string
+	// Copy deep-copies the metric.
 	Copy() Metric
 	// Split will attempt to return multiple metrics with the same timestamp
 	// whose string representations are no longer than maxSize.